Adds the async dispatch infrastructure :
- Postgres pool + embedded migration (CREATE TABLE/INDEX IF NOT EXISTS
gateway_jobs). Auto-applied at boot. lib/pq driver (matches webapp
convention).
- queue.go : Enqueue (idempotent on UNIQUE(bot_slug, update_id) — handles
Telegram redelivery), Pop with FOR UPDATE SKIP LOCKED, MarkDone,
MarkFailed with exponential backoff (30s → 2m → 10m → 1h → dead at 5).
- worker.go : goroutine that drains the queue, dispatches via the same
Handler interface as sync, schedules retries on failure, notifies the
user once when a job goes to dead.
- BotConfig gains `async: bool`. Registry refuses bots with async=true
if DATABASE_URL is unset (queue=nil).
- Server : when bot.Async, the webhook ack is immediate ; the update
payload is enqueued for the worker.
When DATABASE_URL is unset (current default), queue/worker stay disabled
and only sync handlers (echo, http, auth) work — no breaking change to
the running cluster.
Refs ~/.claude/plans/pour-les-notifications-on-inherited-seal.md § Phase 2.
