Phase 2b — durable Postgres queue + worker (gated on DATABASE_URL)

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.

go.mod

@@ -3,6 +3,7 @@ module github.com/arcodange/telegram-gateway
 go 1.24
 
 require (
+	github.com/lib/pq v1.12.3
 	github.com/redis/go-redis/v9 v9.19.0
 	gopkg.in/yaml.v3 v3.0.1
 )