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/services/service_health.py

@@ -0,0 +1,108 @@
+"""
+Background health-checker for registered feature services.
+
+Polls each service's /health endpoint every POLL_INTERVAL seconds and stores
+the result in an in-memory dict.  The REST layer reads from that dict — no DB,
+no blocking calls on the request path.
+"""
+import asyncio
+import logging
+from dataclasses import dataclass, field
+
+import httpx
+
+logger = logging.getLogger(__name__)
+
+POLL_INTERVAL = 30  # seconds
+
+
+@dataclass
+class ServiceDefinition:
+    id: str
+    name: str
+    description: str
+    internal_url: str          # e.g. http://doc-service:8001
+    health_path: str = "/health"
+    app_path: str = ""         # frontend route; empty = no open button
+    settings_path: str = ""    # frontend admin-settings route
+
+
+# ── Registry ──────────────────────────────────────────────────────────────────
+# Add new services here.  The internal_url is filled in at startup from settings.
+
+_REGISTRY: list[ServiceDefinition] = []
+
+# id → True/False/None (None = not yet checked)
+_health: dict[str, bool | None] = {}
+
+
+def register_services(doc_service_url: str, ai_service_url: str) -> None:
+    """Called once during app startup to populate the registry from config."""
+    global _REGISTRY, _health
+
+    _REGISTRY = [
+        ServiceDefinition(
+            id="doc-service",
+            name="Documents",
+            description="Upload PDF files, extract data, and organise them with categories.",
+            internal_url=doc_service_url,
+            health_path="/health",
+            app_path="/apps/documents",
+            settings_path="/apps/documents/settings/admin",
+        ),
+        ServiceDefinition(
+            id="ai-service",
+            name="AI Service",
+            description="Shared AI provider for all features. Configure model, credentials, and connection.",
+            internal_url=ai_service_url,
+            health_path="/health",
+            app_path="",
+            settings_path="/apps/ai/settings/admin",
+        ),
+    ]
+
+    _health = {svc.id: None for svc in _REGISTRY}
+
+
+# ── Health check logic ────────────────────────────────────────────────────────
+
+
+async def _check_service(svc: ServiceDefinition) -> None:
+    url = f"{svc.internal_url}{svc.health_path}"
+    try:
+        async with httpx.AsyncClient(timeout=5.0) as client:
+            resp = await client.get(url)
+        _health[svc.id] = resp.status_code == 200
+    except Exception:
+        _health[svc.id] = False
+
+
+async def check_all() -> None:
+    """Run health checks for all registered services concurrently."""
+    await asyncio.gather(*[_check_service(svc) for svc in _REGISTRY])
+
+
+async def health_check_loop() -> None:
+    """Runs forever; polls every POLL_INTERVAL seconds."""
+    while True:
+        await check_all()
+        await asyncio.sleep(POLL_INTERVAL)
+
+
+# ── Public read API ───────────────────────────────────────────────────────────
+
+
+def get_all_statuses() -> list[dict]:
+    """Return the current health snapshot for all registered services."""
+    return [
+        {
+            "id": svc.id,
+            "name": svc.name,
+            "description": svc.description,
+            "app_path": svc.app_path,
+            "settings_path": svc.settings_path,
+            # None means not yet checked; treat as unhealthy for the UI
+            "healthy": bool(_health.get(svc.id)),
+        }
+        for svc in _REGISTRY
+    ]