feat: gamification queue with deduplication and unpublish revoke

- Add migration 0004: partial unique index on user_points (user_id, action, recording_id)
  for RECORDING_CREATE and RECORDING_FEATURED to prevent earn-on-republish farming
- Add revokePoints() to gamification lib; awardPoints() now uses onConflictDoNothing
- Add gamificationQueue (BullMQ) with 3-attempt exponential backoff
- Add gamification worker handling awardPoints, revokePoints, checkAchievements jobs
- Move all inline gamification calls in recordings + comments resolvers to queue
- Revoke RECORDING_CREATE points when a recording is unpublished (published → draft)
- Register gamification worker at server startup alongside mail worker

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

packages/backend/src/migrations/0004_gamification_unique.sql

@@ -0,0 +1,6 @@
+-- Partial unique index: prevents duplicate RECORDING_CREATE / RECORDING_FEATURED points
+-- for the same recording. RECORDING_PLAY / RECORDING_COMPLETE are excluded so a user
+-- can earn play points across multiple sessions.
+CREATE UNIQUE INDEX "user_points_unique_action_recording"
+ON "user_points" ("user_id", "action", "recording_id")
+WHERE "action" IN ('RECORDING_CREATE', 'RECORDING_FEATURED') AND "recording_id" IS NOT NULL;