feat(worker-pool): Registry-1A scaffold — worker_capabilities/heartbeats + /internal/worker/* 5 endpoint 503 stub

PR-Worker-Pool-Registry-1A (scaffold only, no runtime activation).

신규:
- migrations/270~274 (1 statement/1 file 강제): worker_capabilities + 2 idx + worker_heartbeats + 1 idx
- app/models/worker_pool.py: WorkerCapability + WorkerHeartbeat ORM (queue.py 패턴)
- app/api/internal_worker.py: 5 endpoint 모두 _stub_503() — register/heartbeat/claim/result/drain
- tests/test_internal_worker_stub.py: 503 응답 smoke (inline ASGI client, DB 의존 0)

수정:
- app/main.py: import + include_router 각 1줄 (prefix=/internal/worker, internal_study 일관)

scaffold-first + phase-gate-material-first 강제 (worker-pool-policy §1, §12):
- 인증 dependency 0 (1B 에서 JWT + require_worker_user)
- ProcessingQueue 변경 0 (방향 b: worker_jobs 별 table = 1B)
- LLM 호출 0 / canonical DB 변경 0 / 운영 자동 분기 0

회귀 0 (1주 안전망 = app/main.py.pre-registry-1a.20260518).

plan: ~/.claude/plans/floofy-exploring-mitten.md

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

app/api/internal_worker.py

@@ -0,0 +1,49 @@
+"""PR-Worker-Pool-Registry-1A scaffold: /internal/worker/* 라우트군 503 stub.
+
+worker-pool-policy §8 의 5개 라우트 (register/heartbeat/claim/result/drain) 자리잡기.
+실 동작 = PR-Worker-Pool-Registry-1B (laptop-worker-bot user + worker_jobs table + recap).
+
+1A 시점에는:
+  - 인증 dependency 없음 (503 first response 라 attack surface 0)
+  - Pydantic schema 없음 (1B 활성화 시 추가)
+  - 모든 endpoint = HTTP 503 + detail
+"""
+
+from fastapi import APIRouter, HTTPException, status
+
+router = APIRouter()
+
+
+def _stub_503(endpoint: str) -> None:
+    raise HTTPException(
+        status_code=status.HTTP_503_SERVICE_UNAVAILABLE,
+        detail=(
+            f"/internal/worker/{endpoint} disabled "
+            "(Registry-1A stub; activates in Registry-1B)"
+        ),
+    )
+
+
+@router.post("/register")
+async def register():
+    _stub_503("register")
+
+
+@router.post("/heartbeat")
+async def heartbeat():
+    _stub_503("heartbeat")
+
+
+@router.post("/claim")
+async def claim():
+    _stub_503("claim")
+
+
+@router.post("/result")
+async def result():
+    _stub_503("result")
+
+
+@router.post("/drain")
+async def drain():
+    _stub_503("drain")