Make the platform list shared across users; keep games private

Platforms no longer have a user_id: there's one global list, unique by
name, that every authenticated user reads from and writes to. Games are
unchanged - still private per user, still referencing platform_id.

- Migration: dropped platform.user_id and its FK, UNIQUE(user_id, name) ->
  UNIQUE(name), index on change_seq alone instead of (user_id, change_seq).
  Amended the one init migration in place rather than stacking a second one,
  since nothing is deployed yet.
- db/query.sql: platform queries no longer take a user_id parameter.
  DeletePlatform's NOT EXISTS guard already checked games from any user (it
  never filtered the subquery by user_id), so deleting a shared platform is
  correctly blocked by any user's games on it, not just the deleter's.
- routes.go: platform handlers still require auth (the list isn't public to
  anonymous requests) but no longer scope by user.
- Tests: updated the mocks for the new signatures, added
  TestListPlatformsIsSharedAcrossUsers.

Verified against a real Postgres: user A creates "PC", user B sees it
immediately, both can add games against it, each only sees their own
games, deleting the platform is blocked while either user has a game on
it, and the name stays globally unique.

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

internal/api/routes_test.go

@@ -119,10 +119,10 @@ func TestDeletePlatformConflictWhenReferencedByGames(t *testing.T) {
 	// the 0-rows result itself and turn it into 409, not just pass through
 	// whatever error (if any) the query returns.
 	mockDB := authedMock()
-	mockDB.GetPlatformFunc = func(ctx context.Context, arg db.GetPlatformParams) (db.GetPlatformRow, error) {
-		return db.GetPlatformRow{ID: arg.ID, Name: "PC"}, nil
+	mockDB.GetPlatformFunc = func(ctx context.Context, id uuid.UUID) (db.GetPlatformRow, error) {
+		return db.GetPlatformRow{ID: id, Name: "PC"}, nil
 	}
-	mockDB.DeletePlatformFunc = func(ctx context.Context, arg db.DeletePlatformParams) (int64, error) {
+	mockDB.DeletePlatformFunc = func(ctx context.Context, id uuid.UUID) (int64, error) {
 		return 0, nil
 	}
 
@@ -135,7 +135,7 @@ func TestDeletePlatformConflictWhenReferencedByGames(t *testing.T) {
 
 func TestDeletePlatformNotFound(t *testing.T) {
 	mockDB := authedMock()
-	mockDB.GetPlatformFunc = func(ctx context.Context, arg db.GetPlatformParams) (db.GetPlatformRow, error) {
+	mockDB.GetPlatformFunc = func(ctx context.Context, id uuid.UUID) (db.GetPlatformRow, error) {
 		return db.GetPlatformRow{}, pgx.ErrNoRows
 	}
 
@@ -161,9 +161,8 @@ func TestCreateGameRejectsScoreOutOfRange(t *testing.T) {
 
 func TestCreatePlatform(t *testing.T) {
 	mockDB := authedMock()
-	mockDB.CreatePlatformFunc = func(ctx context.Context, arg db.CreatePlatformParams) (db.CreatePlatformRow, error) {
-		assert.Equal(t, testUserID, arg.UserID)
-		return db.CreatePlatformRow{ID: uuid.New(), Name: arg.Name}, nil
+	mockDB.CreatePlatformFunc = func(ctx context.Context, name string) (db.CreatePlatformRow, error) {
+		return db.CreatePlatformRow{ID: uuid.New(), Name: name}, nil
 	}
 
 	e, _ := newTestAPI(mockDB)
@@ -174,6 +173,27 @@ func TestCreatePlatform(t *testing.T) {
 	assert.Contains(t, rec.Body.String(), "Sega Genesis")
 }
 
+func TestListPlatformsIsSharedAcrossUsers(t *testing.T) {
+	// Unlike games, the platform list has no user_id to scope by: every
+	// authenticated user sees the same rows. ListPlatformsFunc taking no
+	// user id (beyond ctx) is itself part of what's being checked here -
+	// this test would fail to compile if a user_id parameter crept back in.
+	calls := 0
+	mockDB := authedMock()
+	mockDB.ListPlatformsFunc = func(ctx context.Context) ([]db.ListPlatformsRow, error) {
+		calls++
+		return []db.ListPlatformsRow{{ID: uuid.New(), Name: "PC"}}, nil
+	}
+
+	e, _ := newTestAPI(mockDB)
+
+	rec := httptest.NewRecorder()
+	e.ServeHTTP(rec, authedRequest(http.MethodGet, "/api/platforms", ""))
+	assert.Equal(t, http.StatusOK, rec.Code)
+	assert.Contains(t, rec.Body.String(), "PC")
+	assert.Equal(t, 1, calls)
+}
+
 func TestRegisterDuplicateEmailConflicts(t *testing.T) {
 	mockDB := &MockQuerier{
 		CreateUserFunc: func(ctx context.Context, arg db.CreateUserParams) (db.CreateUserRow, error) {