store: add User type and ProvisionUser

Lays the groundwork for per-user accounts: CreateUser/GetUserBySub/
ListUsers plus ProvisionUser, which seeds a brand-new user's profile,
default workout-kind taxonomy, and sync state in one transaction. Depends
on later tasks scoping GetProfile/ListWorkoutKinds/GetSyncState to compile
and pass -- expected or committing to a shared branch.

backend/internal/store/users_test.go

@@ -0,0 +1,91 @@
+package store
+
+import (
+	"context"
+	"testing"
+)
+
+func TestGetUserBySub_NotFoundReturnsFalseNotError(t *testing.T) {
+	db := openTestDB(t)
+	_, found, err := db.GetUserBySub(context.Background(), "no-such-sub")
+	if err != nil {
+		t.Fatalf("GetUserBySub: %v", err)
+	}
+	if found {
+		t.Fatal("expected found=false for an unknown sub")
+	}
+}
+
+func TestProvisionUser_SeedsProfileTaxonomyAndSyncState(t *testing.T) {
+	db := openTestDB(t)
+	ctx := context.Background()
+
+	userID, err := db.ProvisionUser(ctx, "sub-123", "Lucie")
+	if err != nil {
+		t.Fatalf("ProvisionUser: %v", err)
+	}
+
+	u, found, err := db.GetUserBySub(ctx, "sub-123")
+	if err != nil || !found {
+		t.Fatalf("GetUserBySub: found=%v err=%v", found, err)
+	}
+	if u.ID != userID || u.DisplayName != "Lucie" {
+		t.Fatalf("got %+v, want ID=%d DisplayName=Lucie", u, userID)
+	}
+
+	profile, err := db.GetProfile(ctx, userID)
+	if err != nil {
+		t.Fatalf("GetProfile: %v", err)
+	}
+	if profile.Name != "Lucie" {
+		t.Errorf("profile.Name = %q, want %q", profile.Name, "Lucie")
+	}
+	if profile.RollingWindowDays != 90 {
+		t.Errorf("profile.RollingWindowDays = %d, want 90 (default)", profile.RollingWindowDays)
+	}
+
+	kinds, err := db.ListWorkoutKinds(ctx, userID, false)
+	if err != nil {
+		t.Fatalf("ListWorkoutKinds: %v", err)
+	}
+	if len(kinds) != 8 {
+		t.Fatalf("expected 8 seeded workout kinds, got %d", len(kinds))
+	}
+
+	state, err := db.GetSyncState(ctx, userID)
+	if err != nil {
+		t.Fatalf("GetSyncState: %v", err)
+	}
+	if state.EarliestSyncedDate != nil || state.BackfillComplete {
+		t.Errorf("expected fresh sync state, got %+v", state)
+	}
+}
+
+func TestProvisionUser_TwoUsersGetIndependentTaxonomies(t *testing.T) {
+	db := openTestDB(t)
+	ctx := context.Background()
+
+	userA, err := db.ProvisionUser(ctx, "sub-a", "A")
+	if err != nil {
+		t.Fatalf("ProvisionUser(a): %v", err)
+	}
+	userB, err := db.ProvisionUser(ctx, "sub-b", "B")
+	if err != nil {
+		t.Fatalf("ProvisionUser(b): %v", err)
+	}
+
+	kindsA, err := db.ListWorkoutKinds(ctx, userA, false)
+	if err != nil {
+		t.Fatalf("ListWorkoutKinds(a): %v", err)
+	}
+	kindsB, err := db.ListWorkoutKinds(ctx, userB, false)
+	if err != nil {
+		t.Fatalf("ListWorkoutKinds(b): %v", err)
+	}
+	if len(kindsA) != 8 || len(kindsB) != 8 {
+		t.Fatalf("expected 8 kinds each, got a=%d b=%d", len(kindsA), len(kindsB))
+	}
+	if kindsA[0].ID == kindsB[0].ID {
+		t.Fatal("expected each user's seeded kinds to be distinct rows")
+	}
+}