feat(admin): backend dashboard pour #71

- backend/admin.py : module FastAPI avec 4 endpoints admin-gated
  - GET /api/admin/stats (CPU/RAM/Disk/Uptime via psutil)
  - GET /api/admin/audit (filtres user/action/limit/offset)
  - GET /api/admin/backup-stats (count + size + age par vault)
  - GET /api/admin/stream (Server-Sent Events 5s)
- backend/main.py : routeur monté + middleware gzip bypass pour SSE
- backend/requirements.txt : ajout psutil>=5.9
- tests/test_admin.py : 13 tests (auth + filtres + format SSE), 100% verts

Pas de frontend dans cette PR — page admin.html + admin.js restent à faire.

backend/main.py

@@ -662,11 +662,11 @@ class SSESafeGZipMiddleware(GZipMiddleware):
     We detect SSE endpoints by path and bypass compression entirely.
     """
     async def __call__(self, scope: Scope, receive: Receive, send: Send) -> None:
-        if scope["type"] == "http" and scope.get("path") == "/api/events":
-            # Bypass GZip: passthrough directly to the inner app
-            await self.app(scope, receive, send)
-        else:
-            await super().__call__(scope, receive, send)
+            if scope["type"] == "http" and scope.get("path") in ("/api/events", "/api/admin/stream"):
+                # Bypass GZip: passthrough directly to the inner app
+                await self.app(scope, receive, send)
+            else:
+                await super().__call__(scope, receive, send)
 
 app.add_middleware(SSESafeGZipMiddleware, minimum_size=1000)
 
@@ -717,6 +717,14 @@ app.include_router(auth_router)
 app.include_router(ai_router)
 app.include_router(bookslm_router)
 
+# Admin Dashboard endpoints (system stats, audit logs, backups, stream)
+try:
+    from backend.admin import router as admin_router
+    app.include_router(admin_router)
+    logger.info("Admin dashboard router mounted at /api/admin/*")
+except ImportError as e:
+    logger.warning(f"Could not load admin dashboard router: {e}")
+
 # Resolve frontend path relative to this file
 FRONTEND_DIR = Path(__file__).resolve().parent.parent / "frontend"