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.sum

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 github.com/davecgh/go-spew v1.1.1/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
 github.com/klauspost/cpuid/v2 v2.2.10 h1:tBs3QSyvjDyFTq3uoc/9xFpCuOsJQFNPiAhYdw2skhE=
 github.com/klauspost/cpuid/v2 v2.2.10/go.mod h1:hqwkgyIinND0mEev00jJYCxPNVRVXFQeu1XKlok6oO0=
+github.com/lib/pq v1.12.3 h1:tTWxr2YLKwIvK90ZXEw8GP7UFHtcbTtty8zsI+YjrfQ=
+github.com/lib/pq v1.12.3/go.mod h1:/p+8NSbOcwzAEI7wiMXFlgydTwcgTr3OSKMsD2BitpA=
 github.com/pmezard/go-difflib v1.0.0 h1:4DBwDE0NGyQoBHbLQYPwSUPoCMWR5BEzIk/f1lZbAQM=
 github.com/pmezard/go-difflib v1.0.0/go.mod h1:iKH77koFhYxTK1pcRnkKkqfTogsbg7gZNVY4sRDYZ/4=
 github.com/redis/go-redis/v9 v9.19.0 h1:XPVaaPSnG6RhYf7p+rmSa9zZfeVAnWsH5h3lxthOm/k=