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>

db/migrations/20261003000000_init.up.sql

@@ -28,17 +28,18 @@ CREATE TABLE sessions (
 
 CREATE INDEX idx_sessions_user_id ON sessions(user_id);
 
+-- Platforms are shared: every user picks from and adds to the same list,
+-- unlike games, which are private per user_id.
 CREATE TABLE platform (
   id         uuid        PRIMARY KEY DEFAULT gen_random_uuid(),
-  user_id    uuid        NOT NULL REFERENCES users(id) ON DELETE CASCADE,
   name       text        NOT NULL,
   updated_at timestamptz NOT NULL DEFAULT now(),
   deleted_at timestamptz,
   change_seq bigint      NOT NULL DEFAULT nextval('change_seq_seq'),
-  UNIQUE (user_id, name)
+  UNIQUE (name)
 );
 
-CREATE INDEX idx_platform_user_change_seq ON platform(user_id, change_seq);
+CREATE INDEX idx_platform_change_seq ON platform(change_seq);
 
 CREATE TABLE game (
   id           uuid        PRIMARY KEY DEFAULT gen_random_uuid(),