fix: resolve all 64 mypy errors across backend

- Add dict[str, Any] / list[Any] type params throughout (fingerprint, models, schemas, scanner, status_checker)
- Add return type annotations to all route functions (nodes, edges, canvas, scan, auth, status, main)
- Fix no-any-return in security.py: cast pwd/jwt results to bool/str explicitly
- Fix canvas.py: use model_validate() for NodeResponse/EdgeResponse, rename db_node/db_edge upsert vars
- Fix scheduler.py: rename 'result' → 'check_result' to avoid type collision
- Fix get_db() return type: AsyncGenerator[AsyncSession, None]
- Add types-PyYAML for yaml import stubs
- Fix scanner.py: remove unnecessary type: ignore comment (nmap has stubs)
- Fix scan.py: wrap scalars().all() with list() for Sequence→list compatibility

backend/app/api/routes/auth.py

@@ -26,7 +26,7 @@ def _load_credentials() -> tuple[str, str]:
 
 
 @router.post("/login", response_model=TokenResponse)
-async def login(body: LoginRequest):
+async def login(body: LoginRequest) -> TokenResponse:
     username, password_hash = _load_credentials()
     if body.username != username or not verify_password(body.password, password_hash):
         raise HTTPException(status_code=status.HTTP_401_UNAUTHORIZED, detail="Invalid credentials")