Merge backfill into Sync now, replace Full backfill with a destructive Reset all

Sync now now runs Backfill (resumes from the watermark, so a widened history
horizon is picked up automatically) then IncrementalSync then detail-fill, in
one click. "Full backfill" is gone -- in its place, "Reset all" (styled as a
destructive action, gated by a confirm dialog) wipes every synced activity
and its laps/kind assignments and rewinds the backfill watermark, so the next
sync performs a genuinely fresh pull instead of trying to patch up existing
rows with newer schema fields.

backend/internal/api/api_test.go

@@ -335,6 +335,42 @@ func TestListActivities_ReportsLockedForManualAndRaceAssignments(t *testing.T) {
 	}
 }
 
+func TestSyncReset_DeletesActivitiesAndBackfillEndpointIsGone(t *testing.T) {
+	s, db := newTestServer(t)
+	ctx := newCtx()
+
+	if _, err := db.UpsertActivity(ctx, store.Activity{GarminActivityID: 1, ActivityType: "running", StartTimeUTC: "2026-07-11 06:00:00", RawJSON: "{}"}); err != nil {
+		t.Fatalf("UpsertActivity: %v", err)
+	}
+
+	router := s.Router()
+	rec := doJSON(t, router, http.MethodPost, "/api/sync/reset", nil)
+	if rec.Code != http.StatusAccepted {
+		t.Fatalf("reset status = %d, body = %s", rec.Code, rec.Body.String())
+	}
+
+	deadline := time.Now().Add(2 * time.Second)
+	for {
+		activities, err := db.ListActivities(ctx, store.ActivityFilter{})
+		if err != nil {
+			t.Fatalf("ListActivities: %v", err)
+		}
+		if len(activities) == 0 {
+			break
+		}
+		if time.Now().After(deadline) {
+			t.Fatalf("timed out waiting for reset to delete activities, still have %d", len(activities))
+		}
+		time.Sleep(10 * time.Millisecond)
+	}
+
+	// "Full backfill" no longer exists as an endpoint -- superseded by reset.
+	rec = doJSON(t, router, http.MethodPost, "/api/sync/backfill", nil)
+	if rec.Code != http.StatusNotFound {
+		t.Errorf("/api/sync/backfill status = %d, want 404 (removed)", rec.Code)
+	}
+}
+
 func TestProgression_ReturnsSortedTimeSeries(t *testing.T) {
 	s, db := newTestServer(t)
 	ctx := newCtx()

backend/internal/api/server.go

@@ -53,7 +53,7 @@ func (s *Server) Router() http.Handler {
 
 		r.Route("/sync", func(r chi.Router) {
 			r.Post("/run", s.handleSyncRun)
-			r.Post("/backfill", s.handleSyncBackfill)
+			r.Post("/reset", s.handleSyncReset)
 			r.Get("/runs", s.handleSyncRuns)
 			r.Get("/status", s.handleSyncStatus)
 		})

backend/internal/api/sync.go

@@ -10,8 +10,17 @@ import (
 // internal/sync.Service.FillPendingDetails.
 const detailFillBatchSize = 50
 
+// handleSyncRun does a full sync pass: Backfill first (resumes from the
+// watermark, so widening the configured history horizon between clicks is
+// picked up automatically), then IncrementalSync (catches anything new since
+// the latest known activity), then fills in details for whatever's still
+// missing them. Activities already fully processed are left untouched --
+// see internal/sync.Service.FillPendingDetails.
 func (s *Server) handleSyncRun(w http.ResponseWriter, r *http.Request) {
 	ok := s.backgroundSync(func(ctx context.Context) error {
+		if err := s.Sync.Backfill(ctx); err != nil {
+			return err
+		}
 		if err := s.Sync.IncrementalSync(ctx); err != nil {
 			return err
 		}
@@ -24,12 +33,13 @@ func (s *Server) handleSyncRun(w http.ResponseWriter, r *http.Request) {
 	writeJSON(w, http.StatusAccepted, map[string]string{"status": "started"})
 }
 
-func (s *Server) handleSyncBackfill(w http.ResponseWriter, r *http.Request) {
+// handleSyncReset wipes every synced activity (and its laps/samples/kind
+// assignments) and rewinds the backfill watermark, so the next Sync Now
+// performs a genuinely fresh pull from Garmin. Destructive -- the frontend
+// gates this behind a confirmation.
+func (s *Server) handleSyncReset(w http.ResponseWriter, r *http.Request) {
 	ok := s.backgroundSync(func(ctx context.Context) error {
-		if err := s.Sync.Backfill(ctx); err != nil {
-			return err
-		}
-		return s.Sync.FillPendingDetails(ctx, detailFillBatchSize)
+		return s.Sync.ResetAll(ctx)
 	})
 	if !ok {
 		writeError(w, http.StatusConflict, "a sync is already in progress")

backend/internal/store/reset.go

@@ -0,0 +1,27 @@
+package store
+
+import (
+	"context"
+	"fmt"
+)
+
+// ResetAllSyncedData deletes every synced activity (cascading to its laps,
+// activity_samples, and kind_assignments via ON DELETE CASCADE) and rewinds
+// the backfill watermark to its initial state, so a subsequent Backfill
+// starts a genuinely fresh pull instead of thinking history is already
+// covered. Workout kinds (the user's taxonomy) are left untouched.
+func (db *DB) ResetAllSyncedData(ctx context.Context) error {
+	tx, err := db.BeginTx(ctx, nil)
+	if err != nil {
+		return fmt.Errorf("begin reset tx: %w", err)
+	}
+	defer tx.Rollback()
+
+	if _, err := tx.ExecContext(ctx, `DELETE FROM activities`); err != nil {
+		return fmt.Errorf("delete activities: %w", err)
+	}
+	if _, err := tx.ExecContext(ctx, `UPDATE sync_state SET earliest_synced_date = NULL, backfill_complete = 0 WHERE id = 1`); err != nil {
+		return fmt.Errorf("reset sync state: %w", err)
+	}
+	return tx.Commit()
+}

backend/internal/store/reset_test.go

@@ -0,0 +1,71 @@
+package store
+
+import (
+	"context"
+	"testing"
+)
+
+func TestResetAllSyncedData_DeletesActivitiesCascadesAndRewindsWatermark(t *testing.T) {
+	db := openTestDB(t)
+	ctx := context.Background()
+
+	activityID, err := db.UpsertActivity(ctx, Activity{GarminActivityID: 1, ActivityType: "running", StartTimeUTC: "2026-07-11 06:00:00", RawJSON: "{}"})
+	if err != nil {
+		t.Fatalf("UpsertActivity: %v", err)
+	}
+	kindID, err := db.CreateWorkoutKind(ctx, WorkoutKind{Name: "Test Reset Kind", RuleJSON: `{"match":"all","conditions":[]}`, IsActive: true})
+	if err != nil {
+		t.Fatalf("CreateWorkoutKind: %v", err)
+	}
+	if err := db.ReplaceLaps(ctx, activityID, []Lap{{LapIndex: 1}}); err != nil {
+		t.Fatalf("ReplaceLaps: %v", err)
+	}
+	if _, err := db.InsertKindAssignment(ctx, KindAssignment{
+		ActivityID: activityID, WorkoutKindID: &kindID, AssignmentSource: AssignmentSourceRuleEngine,
+		Status: AssignmentStatusAssigned, CandidateKindsJSON: "[]",
+	}); err != nil {
+		t.Fatalf("InsertKindAssignment: %v", err)
+	}
+	if err := db.UpdateSyncState(ctx, "2020-01-01", true); err != nil {
+		t.Fatalf("UpdateSyncState: %v", err)
+	}
+
+	if err := db.ResetAllSyncedData(ctx); err != nil {
+		t.Fatalf("ResetAllSyncedData: %v", err)
+	}
+
+	activities, err := db.ListActivities(ctx, ActivityFilter{})
+	if err != nil {
+		t.Fatalf("ListActivities: %v", err)
+	}
+	if len(activities) != 0 {
+		t.Errorf("expected 0 activities after reset, got %d", len(activities))
+	}
+	laps, err := db.LapsForActivity(ctx, activityID)
+	if err != nil {
+		t.Fatalf("LapsForActivity: %v", err)
+	}
+	if len(laps) != 0 {
+		t.Errorf("expected laps to cascade-delete, got %d", len(laps))
+	}
+	if _, ok, err := db.CurrentAssignment(ctx, activityID); err != nil || ok {
+		t.Errorf("expected kind assignment to cascade-delete, ok=%v err=%v", ok, err)
+	}
+
+	state, err := db.GetSyncState(ctx)
+	if err != nil {
+		t.Fatalf("GetSyncState: %v", err)
+	}
+	if state.EarliestSyncedDate != nil || state.BackfillComplete {
+		t.Errorf("expected watermark rewound to fresh state, got %+v", state)
+	}
+
+	// Workout kinds (the user's taxonomy) must survive a reset.
+	kind, ok, err := db.GetWorkoutKind(ctx, kindID)
+	if err != nil || !ok {
+		t.Fatalf("GetWorkoutKind after reset: ok=%v err=%v", ok, err)
+	}
+	if kind.Name != "Test Reset Kind" {
+		t.Errorf("workout kind was unexpectedly affected by reset: %+v", kind)
+	}
+}

backend/internal/sync/service.go

@@ -203,6 +203,14 @@ func (s *Service) IncrementalSync(ctx context.Context) error {
 	return s.db.FinishSyncRun(ctx, runID, n, nil)
 }
 
+// ResetAll deletes every synced activity (and its laps/samples/kind
+// assignments) and rewinds the backfill watermark, so the next Backfill
+// call performs a genuinely fresh pull from Garmin instead of resuming from
+// wherever the previous one left off. Workout kinds are left untouched.
+func (s *Service) ResetAll(ctx context.Context) error {
+	return s.db.ResetAllSyncedData(ctx)
+}
+
 func (s *Service) fetchAndStoreWindow(ctx context.Context, startDate, endDate string) (int, error) {
 	activities, err := s.garmin.GetActivities(ctx, startDate, endDate, 500)
 	if err != nil {

backend/internal/sync/service_test.go

@@ -427,6 +427,47 @@ func TestBackfill_SecondRunIsANoOpOnceHorizonFullyCovered(t *testing.T) {
 	}
 }
 
+func TestResetAll_AllowsFreshBackfillAfterwards(t *testing.T) {
+	db := openTestDB(t)
+	ctx := context.Background()
+
+	m := &mock.Client{Activities: []garmin.Activity{
+		{ActivityID: 1, ActivityType: garmin.ActivityType{TypeKey: "running"}, StartTimeGMT: "2026-07-05 06:00:00", Distance: 5000, Duration: 1500},
+	}}
+	svc := NewService(m, db, Config{BackfillHorizonDays: 10, BackfillWindowDays: 10},
+		fixedNow(time.Date(2026, 7, 11, 0, 0, 0, 0, time.UTC)))
+
+	if err := svc.Backfill(ctx); err != nil {
+		t.Fatalf("first Backfill: %v", err)
+	}
+	firstCallCount := m.GetActivitiesCalls
+
+	if err := svc.ResetAll(ctx); err != nil {
+		t.Fatalf("ResetAll: %v", err)
+	}
+	activities, err := db.ListActivities(ctx, store.ActivityFilter{})
+	if err != nil {
+		t.Fatalf("ListActivities: %v", err)
+	}
+	if len(activities) != 0 {
+		t.Fatalf("expected 0 activities after ResetAll, got %d", len(activities))
+	}
+
+	if err := svc.Backfill(ctx); err != nil {
+		t.Fatalf("Backfill after reset: %v", err)
+	}
+	if m.GetActivitiesCalls <= firstCallCount {
+		t.Errorf("expected Backfill after ResetAll to call GetActivities again (fresh pull), call count stayed at %d", m.GetActivitiesCalls)
+	}
+	activities, err = db.ListActivities(ctx, store.ActivityFilter{})
+	if err != nil {
+		t.Fatalf("ListActivities after re-backfill: %v", err)
+	}
+	if len(activities) != 1 {
+		t.Fatalf("expected 1 activity re-fetched after reset+backfill, got %d", len(activities))
+	}
+}
+
 func TestBackfill_ResumesFromWatermarkWhenHorizonGrows(t *testing.T) {
 	db := openTestDB(t)
 	ctx := context.Background()