refactor: improve code structure and optimize key functions

backend/app/core/deps.py

@@ -0,0 +1,26 @@
+from typing import Annotated, AsyncGenerator
+
+from fastapi import Depends, HTTPException, status
+from pydantic import ValidationError
+from sqlalchemy.ext.asyncio import AsyncSession
+
+from app.core.security import decode_access_token, oauth2_scheme
+from app.db.session import SessionLocal
+from app.schemas.user import TokenPayload
+
+
+async def get_db() -> AsyncGenerator[AsyncSession, None]:
+    async with SessionLocal() as session:
+        yield session
+
+
+async def get_current_user(token: Annotated[str, Depends(oauth2_scheme)]) -> TokenPayload:
+    payload = decode_access_token(token)
+    try:
+        return TokenPayload(**payload)
+    except ValidationError as exc:
+        raise HTTPException(
+            status_code=status.HTTP_401_UNAUTHORIZED,
+            detail="Could not validate credentials",
+            headers={"WWW-Authenticate": "Bearer"},
+        ) from exc