fix: address multiple code quality improvements across backend and frontend

Backend fixes:
- Fix JWT sub claim: encode as str(user.id), decode with int() cast (python-jose requirement)
- Replace all deprecated datetime.utcnow() with datetime.now(timezone.utc)
- Replace deprecated FastAPI @app.on_event() with modern lifespan context manager
- Replace deprecated Pydantic class Config with model_config = ConfigDict(...)
- Replace deprecated Pydantic .dict() with .model_dump()
- Fix overly broad except (GmailInjectionError, Exception) → except Exception
- Remove unused GmailInjectionError import
- Fix TokenPayload schema sub field type from int to str

Frontend:
- Create frontend/src/lib/api.ts — API client module with auth, user, mail accounts, processing runs APIs
- Add !frontend/src/lib/ to .gitignore negation

Tests:
- Add 3 new JWT tests (sub string encoding, access token type, refresh token type)
- Update test_token_payload_schema for string sub claim
- All 128 tests pass

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>
Agent-Logs-Url: https://github.com/christianlouis/pop_puller_to_gmail/sessions/e0b13eb0-8de7-4f02-81e4-e202cbba4608

backend/app/core/security.py

@@ -4,7 +4,7 @@ Security utilities for encryption, hashing, and token generation.
 
 import hashlib
 import secrets
-from datetime import datetime, timedelta
+from datetime import datetime, timedelta, timezone
 from typing import Optional, Dict, Any
 import bcrypt
 from jose import JWTError, jwt
@@ -35,9 +35,9 @@ def create_access_token(
     to_encode = data.copy()
 
     if expires_delta:
-        expire = datetime.utcnow() + expires_delta
+        expire = datetime.now(timezone.utc) + expires_delta
     else:
-        expire = datetime.utcnow() + timedelta(
+        expire = datetime.now(timezone.utc) + timedelta(
             minutes=settings.ACCESS_TOKEN_EXPIRE_MINUTES
         )
 
@@ -51,7 +51,9 @@ def create_access_token(
 def create_refresh_token(data: Dict[str, Any]) -> str:
     """Create JWT refresh token"""
     to_encode = data.copy()
-    expire = datetime.utcnow() + timedelta(days=settings.REFRESH_TOKEN_EXPIRE_DAYS)
+    expire = datetime.now(timezone.utc) + timedelta(
+        days=settings.REFRESH_TOKEN_EXPIRE_DAYS
+    )
     to_encode.update({"exp": expire, "type": "refresh"})
     encoded_jwt = jwt.encode(
         to_encode, settings.SECRET_KEY, algorithm=settings.ALGORITHM