feat: add /api/ready readiness probe (DB, data dir, local-first) (#1200)

/api/health is a liveness ping. This adds /api/ready as a readiness /
integrity self-check that returns 503 unless every critical subsystem is
whole, so an orchestrator (Docker/Compose/k8s) can gate traffic on real
readiness rather than mere process liveness:

  - database: opens a connection and runs SELECT 1
  - data_dir: confirms the data directory exists and is writable
  - local_first: reports whether storage stays on the host (informational;
    a remote database is a valid deployment, so it never fails readiness)

The check logic lives in src/readiness.py so it is unit-testable in
isolation; the route is a thin wrapper. Covered by tests/test_readiness.py.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

app.py

@@ -772,6 +772,17 @@ async def get_version():
 async def health_check() -> Dict[str, str]:
     return {"status": "healthy", "timestamp": datetime.utcnow().isoformat()}
 
+@app.get("/api/ready")
+async def readiness_check() -> JSONResponse:
+    """Readiness / integrity self-check — DB, data dir, local-first storage.
+
+    Unlike /api/health (liveness), this returns 503 unless every critical
+    subsystem is whole, so an orchestrator can gate traffic on real readiness.
+    """
+    from src.readiness import check_readiness
+    result = check_readiness()
+    return JSONResponse(status_code=200 if result.get("ready") else 503, content=result)
+
 @app.get("/api/runtime")
 async def runtime_info() -> Dict[str, object]:
     in_docker = os.path.exists("/.dockerenv")