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.

migrations/001_init.sql

@@ -0,0 +1,24 @@
+-- Phase 2b — durable job queue for async handlers (shell, script, ollama).
+-- Voir ~/.claude/plans/pour-les-notifications-on-inherited-seal.md § Phase 2.
+CREATE TABLE IF NOT EXISTS gateway_jobs (
+  id            BIGSERIAL    PRIMARY KEY,
+  bot_slug      TEXT         NOT NULL,
+  handler_type  TEXT         NOT NULL,
+  update_id     BIGINT       NOT NULL,
+  payload       JSONB        NOT NULL,
+  attempts      INT          NOT NULL DEFAULT 0,
+  next_retry_at TIMESTAMPTZ  NOT NULL DEFAULT NOW(),
+  status        TEXT         NOT NULL DEFAULT 'pending',
+  last_error    TEXT,
+  created_at    TIMESTAMPTZ  NOT NULL DEFAULT NOW(),
+  updated_at    TIMESTAMPTZ  NOT NULL DEFAULT NOW(),
+  -- idempotence : si Telegram redélivre (ack > 60s), on ignore le doublon.
+  CONSTRAINT gateway_jobs_uniq_update UNIQUE (bot_slug, update_id)
+);
+
+CREATE INDEX IF NOT EXISTS gateway_jobs_pickable
+  ON gateway_jobs (next_retry_at)
+  WHERE status = 'pending';
+
+CREATE INDEX IF NOT EXISTS gateway_jobs_status
+  ON gateway_jobs (status);