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/db/querier.go

@@ -12,23 +12,24 @@ import (
 
 type Querier interface {
 	CreateGame(ctx context.Context, arg CreateGameParams) (CreateGameRow, error)
-	CreatePlatform(ctx context.Context, arg CreatePlatformParams) (CreatePlatformRow, error)
+	CreatePlatform(ctx context.Context, name string) (CreatePlatformRow, error)
 	CreateSession(ctx context.Context, arg CreateSessionParams) error
 	CreateUser(ctx context.Context, arg CreateUserParams) (CreateUserRow, error)
 	DeleteGame(ctx context.Context, arg DeleteGameParams) (int64, error)
 	// Soft delete, so the FK on game.platform_id is never touched and never
 	// blocks this on its own; the NOT EXISTS guard is what stops a platform
-	// that still has active games from being deleted. Call GetPlatform first to
-	// tell "not found" apart from "blocked by games" when this affects 0 rows.
-	DeletePlatform(ctx context.Context, arg DeletePlatformParams) (int64, error)
+	// that still has active games - from any user, since the list is shared -
+	// from being deleted. Call GetPlatform first to tell "not found" apart from
+	// "blocked by games" when this affects 0 rows.
+	DeletePlatform(ctx context.Context, id uuid.UUID) (int64, error)
 	DeleteSession(ctx context.Context, tokenHash []byte) error
 	GetGame(ctx context.Context, arg GetGameParams) (GetGameRow, error)
-	GetPlatform(ctx context.Context, arg GetPlatformParams) (GetPlatformRow, error)
+	GetPlatform(ctx context.Context, id uuid.UUID) (GetPlatformRow, error)
 	GetSessionAndUser(ctx context.Context, tokenHash []byte) (GetSessionAndUserRow, error)
 	GetUserByEmail(ctx context.Context, email string) (GetUserByEmailRow, error)
 	GetUserByID(ctx context.Context, id uuid.UUID) (GetUserByIDRow, error)
 	ListGames(ctx context.Context, userID uuid.UUID) ([]ListGamesRow, error)
-	ListPlatforms(ctx context.Context, userID uuid.UUID) ([]ListPlatformsRow, error)
+	ListPlatforms(ctx context.Context) ([]ListPlatformsRow, error)
 	TouchSession(ctx context.Context, tokenHash []byte) error
 	UpdateGame(ctx context.Context, arg UpdateGameParams) (UpdateGameRow, error)
 	UpdatePlatform(ctx context.Context, arg UpdatePlatformParams) (UpdatePlatformRow, error)