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>

features/doc-service/app/main.py

@@ -1,15 +1,45 @@
+import asyncio
+import logging
+from contextlib import asynccontextmanager
+
 from fastapi import FastAPI
 
 from app.core.config import settings
 from app.routers import categories, documents
+from app.routers import plugin as plugin_router
 
-app = FastAPI(title=settings.PROJECT_NAME)
+logger = logging.getLogger(__name__)
+
+
+@asynccontextmanager
+async def lifespan(app: FastAPI):
+    loop = asyncio.get_running_loop()
+    watcher = None
+
+    try:
+        from app.services.config_reader import get_storage_config
+        storage_config = await get_storage_config()
+        if storage_config.get("watch_enabled"):
+            from app.services.file_watcher import FileWatcherService
+            watcher = FileWatcherService(loop)
+            await watcher.start(storage_config["watch_path"], storage_config)
+    except Exception as exc:
+        logger.warning("[doc-service] File watcher could not start: %s", exc)
+
+    yield
+
+    if watcher is not None:
+        await watcher.stop()
+
+
+app = FastAPI(title=settings.PROJECT_NAME, lifespan=lifespan)
 
 # No CORS — this service is only reachable from the main backend on backend-net.
 # All browser traffic goes through the main backend proxy.
 
 app.include_router(documents.router, prefix="/documents", tags=["documents"])
 app.include_router(categories.router, prefix="/categories", tags=["categories"])
+app.include_router(plugin_router.router, prefix="/plugin", tags=["plugin"])
 
 
 @app.get("/health")