Add service health checks and dynamic Apps page

Backend polls each registered service's /health endpoint every 30 s via a
background asyncio task. GET /api/services exposes the live status snapshot.
The Apps page now renders from this endpoint — showing "Unavailable" (dimmed,
non-clickable) when a service is registered but its container is unreachable.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/core/config.py

@@ -15,6 +15,7 @@ class Settings(BaseSettings):
 
     CORS_ORIGINS: list[str] = ["http://localhost:5173"]
 
+    DOC_SERVICE_URL: str = "http://doc-service:8001"
     AI_SERVICE_URL: str = "http://ai-service:8010"
 
     @field_validator("JWT_PRIVATE_KEY", "JWT_PUBLIC_KEY", mode="before")