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/db/database.py

@@ -1,3 +1,4 @@
+from collections.abc import AsyncGenerator
 from contextlib import suppress
 
 from sqlalchemy.ext.asyncio import AsyncSession, async_sessionmaker, create_async_engine
@@ -31,6 +32,6 @@ async def init_db() -> None:
             await conn.exec_driver_sql("ALTER TABLE edges ADD COLUMN path_style TEXT")
 
 
-async def get_db():
+async def get_db() -> AsyncGenerator[AsyncSession, None]:
     async with AsyncSessionLocal() as session:
         yield session