store: scope workout kinds and their pace/HR ranges to a user_id

workout_kinds gains a real user_id column (Task 1); workout_type_paces has
none of its own and is scoped via a join to workout_kinds instead, since
it's always accessed through a specific kind.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/internal/store/reset_test.go

@@ -65,7 +65,7 @@ func TestResetAllSyncedData_DeletesActivitiesCascadesAndRewindsWatermark(t *test
 	}
 
 	// Workout kinds (the user's taxonomy) must survive a reset.
-	kind, ok, err := db.GetWorkoutKind(ctx, kindID)
+	kind, ok, err := db.GetWorkoutKind(ctx, userID, kindID)
 	if err != nil || !ok {
 		t.Fatalf("GetWorkoutKind after reset: ok=%v err=%v", ok, err)
 	}