feat(store): add DeleteUser with cascading account deletion

backend/internal/store/isolation_test.go

@@ -161,3 +161,46 @@ func TestIsolation_SyncStateAndRunsNeverLeakAcrossUsers(t *testing.T) {
 		t.Fatal("userB's FinishSyncRun call was able to mutate userA's sync run")
 	}
 }
+
+// TestIsolation_DeleteUserLeavesOtherUsersDataIntact confirms deleting one
+// user's account never touches another user's profile, taxonomy, or
+// activities, even though DeleteUser is a single blunt DELETE FROM users.
+func TestIsolation_DeleteUserLeavesOtherUsersDataIntact(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)
+	}
+	if _, err := db.UpsertActivity(ctx, userB, Activity{GarminActivityID: 1, StartTimeUTC: "2026-07-11 06:00:00", RawJSON: "{}"}); err != nil {
+		t.Fatalf("UpsertActivity(b): %v", err)
+	}
+
+	if err := db.DeleteUser(ctx, userA); err != nil {
+		t.Fatalf("DeleteUser(a): %v", err)
+	}
+
+	if _, found, err := db.GetUserBySub(ctx, "sub-b"); err != nil || !found {
+		t.Fatalf("expected userB to survive userA's deletion, found=%v err=%v", found, err)
+	}
+	profileB, err := db.GetProfile(ctx, userB)
+	if err != nil {
+		t.Fatalf("GetProfile(b) after deleting a: %v", err)
+	}
+	if profileB.Name != "B" {
+		t.Errorf("userB's profile changed after deleting userA: %+v", profileB)
+	}
+	kindsB, err := db.ListWorkoutKinds(ctx, userB, false)
+	if err != nil || len(kindsB) != 8 {
+		t.Fatalf("userB's taxonomy affected by deleting userA: len=%d err=%v", len(kindsB), err)
+	}
+	activitiesB, err := db.ListActivities(ctx, userB, ActivityFilter{})
+	if err != nil || len(activitiesB) != 1 {
+		t.Fatalf("userB's activities affected by deleting userA: len=%d err=%v", len(activitiesB), err)
+	}
+}