Replace per-kind reclassify with a single global reclassify that respects manual and Race locks

Manual assignments are the user's definitive word and Race assignments come
from a hard Garmin fact (eventType.typeKey), not a retunable rule -- neither
is ever touched by reclassify again, and Race can no longer be set by hand
via the review queue. Adds an Activities page so this locked/unlocked status
is visible per workout, since Review Queue only ever showed unresolved items.

backend/internal/api/reclassify.go

@@ -0,0 +1,51 @@
+package api
+
+import (
+	"net/http"
+
+	"smartrun/backend/internal/store"
+)
+
+// handleReclassifyAll re-runs the rule engine for every activity except
+// those that are locked:
+//   - a manual assignment is the user's definitive word and is never
+//     overwritten by the rule engine again;
+//   - an activity already assigned to Race is locked too, since that
+//     classification comes from a hard Garmin fact (eventType.typeKey ==
+//     "race"), not a rule the user might retune -- there's nothing to
+//     re-derive.
+//
+// It's a synchronous, bounded operation (unlike sync), so it runs inline
+// rather than in the background.
+func (s *Server) handleReclassifyAll(w http.ResponseWriter, r *http.Request) {
+	raceKind, hasRaceKind, err := s.DB.GetWorkoutKindByName(r.Context(), "Race")
+	if err != nil {
+		writeError(w, http.StatusInternalServerError, err.Error())
+		return
+	}
+
+	assignments, err := s.DB.AllCurrentAssignments(r.Context())
+	if err != nil {
+		writeError(w, http.StatusInternalServerError, err.Error())
+		return
+	}
+
+	var activityIDs []int64
+	for _, a := range assignments {
+		if a.AssignmentSource == store.AssignmentSourceManual {
+			continue
+		}
+		if hasRaceKind && a.WorkoutKindID != nil && *a.WorkoutKindID == raceKind.ID {
+			continue
+		}
+		activityIDs = append(activityIDs, a.ActivityID)
+	}
+
+	for _, activityID := range activityIDs {
+		if err := s.Sync.ClassifyActivity(r.Context(), activityID); err != nil {
+			writeError(w, http.StatusInternalServerError, err.Error())
+			return
+		}
+	}
+	writeJSON(w, http.StatusOK, map[string]int{"reclassified": len(activityIDs)})
+}