feat: add durable queue, single worker, and drain-by-model scheduling

Replace the Phase 2 in-flight chat gate (buffered channel) with a real
SQLite-backed job queue and single worker loop. Every /api/chat request
now creates a job row, blocks until the worker completes it, and returns
the result transparently.

Key changes:
- internal/store: NextJob (drain-by-model ordering), IncrementAttempt,
  ResetInterruptedJobs, DeleteTerminalJobsBefore; busy_timeout pragma
- internal/worker: single-threaded worker loop with Notifier for sync
  handler completion signaling; retry on ConnectionError, terminal fail
  on HTTPError; crash recovery resets interrupted jobs on startup
- internal/webhook: dispatcher infrastructure for async webhook delivery
- internal/server: chat handler rewritten to enqueue+wait; old chatGate
  removed; embeddings remain direct concurrent proxies (ADR-0013)
- internal/config: FOREMAN_MAX_ATTEMPTS, FOREMAN_JOB_TTL

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

go.sum

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