feat(coordinator): worker registry, API contract, logging & DB retry

Align the coordinator with the master PLAN.md (CTX-00, CTX-04) and harden
process startup.

- CTX-00: freeze docs/api-contract.md as the v1 source of truth for the
  Go coordinator and Python worker.
- CTX-04: worker registry — workers table (migration 0002), domain.Worker,
  RegisterWorker use case, WorkerRepository, and POST /workers/register.
- Contract alignment: claim uses `capabilities` (was `workloads`),
  COORDINATOR_TOKEN env (WORKER_AUTH_TOKEN kept as fallback), and
  GET /health now reports database readiness (503 when the DB is down).
- Logging: logs are teed to stdout and an optional rotated file (LOG_FILE)
  via lumberjack, so they survive a container rebuild.
- Startup resilience: the initial DB connection is retried with backoff,
  so the coordinator waits for Postgres to boot instead of crash-looping.

coordinator/scripts/smoke.sh

@@ -45,6 +45,13 @@ check "GET /health"                     200 "${HOST}/health"
 check "claim without a token → 401"     401 -X POST "${HOST}/tasks/claim" \
 	-H 'Content-Type: application/json' -d '{"worker_id":"w1"}'
 
+echo
+echo "worker registry"
+check "register worker"                 201 -X POST "${HOST}/workers/register" "${auth[@]}" \
+	-d '{"name":"smoke-worker","capabilities":["similarity_search"],"cpu_count":4,"memory_mb":8192}'
+check "register without capabilities → 400" 400 -X POST "${HOST}/workers/register" "${auth[@]}" \
+	-d '{"name":"bad"}'
+
 echo
 echo "job lifecycle"
 job=$(curl -sS "${auth[@]}" -X POST "${HOST}/jobs" -d '{