feat: implement event bus backend configuration for DB and NATS

.env.example

@@ -39,17 +39,18 @@ ALLOW_REGISTRATION=false
 RATE_LIMIT=20
 POSTER_FETCH_TIMEOUT_SECONDS=30
 
-# Event bus — controls background poster sync and other async work.
-# Priority: NATS (if NATS_URL set) → DB queue (default, uses existing DB)
+# Event bus — "db" (default) or "nats"
 # The worker binary must run alongside the presentation to process events.
+EVENT_BUS_BACKEND=db
 
 # Option A: DB queue (default — no extra infrastructure needed)
-# Events are stored in the same database as the app and polled by the worker.
+# Events are persisted in the same database as the app and polled by the worker.
 # EVENT_QUEUE_POLL_INTERVAL_MS=500   # polling interval (default: 500ms)
 # EVENT_QUEUE_BATCH_SIZE=10          # rows claimed per poll cycle (default: 10)
 # EVENT_QUEUE_MAX_ATTEMPTS=5         # retries before dead-lettering (default: 5)
 
 # Option B: NATS (at-least-once delivery, recommended for higher throughput)
+# EVENT_BUS_BACKEND=nats
 # NATS_URL=nats://localhost:4222
 # NATS_MODE=jetstream           # "jetstream" (default, at-least-once) or "core" (fire-and-forget)
 # NATS_SUBJECT_PREFIX=movies-diary.events