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/models.go

@@ -24,7 +24,6 @@ type Game struct {
 
 type Platform struct {
 	ID        uuid.UUID
-	UserID    uuid.UUID
 	Name      string
 	UpdatedAt pgtype.Timestamptz
 	DeletedAt pgtype.Timestamptz