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/.env.example

@@ -6,8 +6,15 @@ DATABASE_URL=postgres://scimesh:scimesh@localhost:5432/scimesh?sslmode=disable
 # Shared bearer token every worker must present. Leave empty to disable auth (dev only).
 WORKER_AUTH_TOKEN=change-me
 
+# Logging. LOG_LEVEL: debug|info|warn|error. LOG_FILE empty = stdout only;
+# set a path to also write a size-rotated file (kept across restarts).
+LOG_LEVEL=info
+# LOG_FILE=./logs/coordinator.log
+
 # Optional tuning (defaults shown).
 DB_MAX_CONNS=10
+# How long to keep retrying the initial DB connection while Postgres boots.
+DB_CONNECT_TIMEOUT=30s
 REQUEST_TIMEOUT=15s
 LEASE_DURATION=2m
 DEFAULT_MAX_ATTEMPTS=3