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/mock_db_test.go

@@ -15,19 +15,19 @@ import (
 // pointer deep in sqlc-generated code.
 type MockQuerier struct {
 	CreateGameFunc        func(ctx context.Context, arg db.CreateGameParams) (db.CreateGameRow, error)
-	CreatePlatformFunc    func(ctx context.Context, arg db.CreatePlatformParams) (db.CreatePlatformRow, error)
+	CreatePlatformFunc    func(ctx context.Context, name string) (db.CreatePlatformRow, error)
 	CreateSessionFunc     func(ctx context.Context, arg db.CreateSessionParams) error
 	CreateUserFunc        func(ctx context.Context, arg db.CreateUserParams) (db.CreateUserRow, error)
 	DeleteGameFunc        func(ctx context.Context, arg db.DeleteGameParams) (int64, error)
-	DeletePlatformFunc    func(ctx context.Context, arg db.DeletePlatformParams) (int64, error)
+	DeletePlatformFunc    func(ctx context.Context, id uuid.UUID) (int64, error)
 	DeleteSessionFunc     func(ctx context.Context, tokenHash []byte) error
 	GetGameFunc           func(ctx context.Context, arg db.GetGameParams) (db.GetGameRow, error)
-	GetPlatformFunc       func(ctx context.Context, arg db.GetPlatformParams) (db.GetPlatformRow, error)
+	GetPlatformFunc       func(ctx context.Context, id uuid.UUID) (db.GetPlatformRow, error)
 	GetSessionAndUserFunc func(ctx context.Context, tokenHash []byte) (db.GetSessionAndUserRow, error)
 	GetUserByEmailFunc    func(ctx context.Context, email string) (db.GetUserByEmailRow, error)
 	GetUserByIDFunc       func(ctx context.Context, id uuid.UUID) (db.GetUserByIDRow, error)
 	ListGamesFunc         func(ctx context.Context, userID uuid.UUID) ([]db.ListGamesRow, error)
-	ListPlatformsFunc     func(ctx context.Context, userID uuid.UUID) ([]db.ListPlatformsRow, error)
+	ListPlatformsFunc     func(ctx context.Context) ([]db.ListPlatformsRow, error)
 	TouchSessionFunc      func(ctx context.Context, tokenHash []byte) error
 	UpdateGameFunc        func(ctx context.Context, arg db.UpdateGameParams) (db.UpdateGameRow, error)
 	UpdatePlatformFunc    func(ctx context.Context, arg db.UpdatePlatformParams) (db.UpdatePlatformRow, error)
@@ -36,8 +36,8 @@ type MockQuerier struct {
 func (m *MockQuerier) CreateGame(ctx context.Context, arg db.CreateGameParams) (db.CreateGameRow, error) {
 	return m.CreateGameFunc(ctx, arg)
 }
-func (m *MockQuerier) CreatePlatform(ctx context.Context, arg db.CreatePlatformParams) (db.CreatePlatformRow, error) {
-	return m.CreatePlatformFunc(ctx, arg)
+func (m *MockQuerier) CreatePlatform(ctx context.Context, name string) (db.CreatePlatformRow, error) {
+	return m.CreatePlatformFunc(ctx, name)
 }
 func (m *MockQuerier) CreateSession(ctx context.Context, arg db.CreateSessionParams) error {
 	return m.CreateSessionFunc(ctx, arg)
@@ -48,8 +48,8 @@ func (m *MockQuerier) CreateUser(ctx context.Context, arg db.CreateUserParams) (
 func (m *MockQuerier) DeleteGame(ctx context.Context, arg db.DeleteGameParams) (int64, error) {
 	return m.DeleteGameFunc(ctx, arg)
 }
-func (m *MockQuerier) DeletePlatform(ctx context.Context, arg db.DeletePlatformParams) (int64, error) {
-	return m.DeletePlatformFunc(ctx, arg)
+func (m *MockQuerier) DeletePlatform(ctx context.Context, id uuid.UUID) (int64, error) {
+	return m.DeletePlatformFunc(ctx, id)
 }
 func (m *MockQuerier) DeleteSession(ctx context.Context, tokenHash []byte) error {
 	return m.DeleteSessionFunc(ctx, tokenHash)
@@ -57,8 +57,8 @@ func (m *MockQuerier) DeleteSession(ctx context.Context, tokenHash []byte) error
 func (m *MockQuerier) GetGame(ctx context.Context, arg db.GetGameParams) (db.GetGameRow, error) {
 	return m.GetGameFunc(ctx, arg)
 }
-func (m *MockQuerier) GetPlatform(ctx context.Context, arg db.GetPlatformParams) (db.GetPlatformRow, error) {
-	return m.GetPlatformFunc(ctx, arg)
+func (m *MockQuerier) GetPlatform(ctx context.Context, id uuid.UUID) (db.GetPlatformRow, error) {
+	return m.GetPlatformFunc(ctx, id)
 }
 func (m *MockQuerier) GetSessionAndUser(ctx context.Context, tokenHash []byte) (db.GetSessionAndUserRow, error) {
 	return m.GetSessionAndUserFunc(ctx, tokenHash)
@@ -72,8 +72,8 @@ func (m *MockQuerier) GetUserByID(ctx context.Context, id uuid.UUID) (db.GetUser
 func (m *MockQuerier) ListGames(ctx context.Context, userID uuid.UUID) ([]db.ListGamesRow, error) {
 	return m.ListGamesFunc(ctx, userID)
 }
-func (m *MockQuerier) ListPlatforms(ctx context.Context, userID uuid.UUID) ([]db.ListPlatformsRow, error) {
-	return m.ListPlatformsFunc(ctx, userID)
+func (m *MockQuerier) ListPlatforms(ctx context.Context) ([]db.ListPlatformsRow, error) {
+	return m.ListPlatformsFunc(ctx)
 }
 func (m *MockQuerier) TouchSession(ctx context.Context, tokenHash []byte) error {
 	return m.TouchSessionFunc(ctx, tokenHash)