Add generic plugin architecture and watch-directory feature

Introduces a manifest contract so feature containers self-describe their
settings (JSON Schema + access rules). Backend and frontend gain generic
plugin proxy and dynamic Extensions UI with zero feature-specific code.

Doc-service is the first plugin consumer: exposes /plugin/manifest and
/plugin/settings, adds a watchdog-based file watcher that auto-ingests
PDFs from a mounted directory, maps subfolders to categories, supports
AI-suggested folder/filename (user-confirmed), and enforces a no-remove
policy. Access is gated by is_superuser or doc-service-admin group.

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

backend/app/services/service_health.py

@@ -2,8 +2,9 @@
 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.
+the result in an in-memory dict.  Also fetches /plugin/manifest when available
+and caches it so the plugin proxy can serve it without per-request network calls.
+The REST layer reads from that dict — no DB, no blocking calls on the request path.
 """
 import asyncio
 import logging
@@ -35,10 +36,13 @@ _REGISTRY: list[ServiceDefinition] = []
 # id → True/False/None (None = not yet checked)
 _health: dict[str, bool | None] = {}
 
+# id → plugin manifest dict, or None if the service has no plugin manifest
+_manifests: dict[str, dict | 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
+    global _REGISTRY, _health, _manifests
 
     _REGISTRY = [
         ServiceDefinition(
@@ -62,6 +66,7 @@ def register_services(doc_service_url: str, ai_service_url: str) -> None:
     ]
 
     _health = {svc.id: None for svc in _REGISTRY}
+    _manifests = {svc.id: None for svc in _REGISTRY}
     logger.info("Service registry initialised with %d services", len(_REGISTRY))
 
 
@@ -88,6 +93,25 @@ async def _check_service(svc: ServiceDefinition) -> None:
         else:
             logger.warning("Service %s is now UNHEALTHY", svc.id)
 
+    # Opportunistically fetch plugin manifest when the service is healthy
+    if healthy:
+        await _fetch_manifest(svc)
+
+
+async def _fetch_manifest(svc: ServiceDefinition) -> None:
+    """Try to GET /plugin/manifest from the service; cache result (or None)."""
+    url = f"{svc.internal_url}/plugin/manifest"
+    try:
+        async with httpx.AsyncClient(timeout=5.0) as client:
+            resp = await client.get(url)
+        if resp.status_code == 200:
+            _manifests[svc.id] = resp.json()
+        else:
+            _manifests[svc.id] = None
+    except Exception:
+        # Service doesn't have a plugin manifest — not an error
+        _manifests[svc.id] = None
+
 
 async def check_all() -> None:
     """Run health checks for all registered services concurrently."""
@@ -125,3 +149,16 @@ def get_all_statuses() -> list[dict]:
         }
         for svc in _REGISTRY
     ]
+
+
+def get_cached_manifest(service_id: str) -> dict | None:
+    """Return the cached plugin manifest for a service, or None if unavailable."""
+    return _manifests.get(service_id)
+
+
+def get_service_url(service_id: str) -> str | None:
+    """Return the internal URL for a registered service, or None if unknown."""
+    for svc in _REGISTRY:
+        if svc.id == service_id:
+            return svc.internal_url
+    return None