store: scope GetProfile/UpdateProfile to a user_id

Part of per-user profile isolation: profile rows are no longer a global
singleton, so every read/write requires the caller's userID. This also
requires scoping ListActivities, ListWorkoutKinds, UpsertActivity,
CreateWorkoutKind, GetSyncState, UpdateSyncState, and ResetAllSyncedData
to userID, plus updating all related tests in the store package.

backend/internal/store/profile_test.go

@@ -8,8 +8,12 @@ import (
 func TestProfile_DefaultsThenUpdate(t *testing.T) {
 	db := openTestDB(t)
 	ctx := context.Background()
+	userID, err := db.ProvisionUser(ctx, "test-sub", "Default")
+	if err != nil {
+		t.Fatalf("ProvisionUser: %v", err)
+	}
 
-	p, err := db.GetProfile(ctx)
+	p, err := db.GetProfile(ctx, userID)
 	if err != nil {
 		t.Fatalf("GetProfile: %v", err)
 	}
@@ -64,11 +68,11 @@ func TestProfile_DefaultsThenUpdate(t *testing.T) {
 	p.MinRepresentativePaceSecPerKm = 600
 	p.MinRepresentativeTimeSeconds = 5
 
-	if err := db.UpdateProfile(ctx, p); err != nil {
+	if err := db.UpdateProfile(ctx, userID, p); err != nil {
 		t.Fatalf("UpdateProfile: %v", err)
 	}
 
-	got, err := db.GetProfile(ctx)
+	got, err := db.GetProfile(ctx, userID)
 	if err != nil {
 		t.Fatalf("GetProfile after update: %v", err)
 	}