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/api/v1/endpoints/auth.py

@@ -6,7 +6,7 @@ from fastapi import APIRouter, Depends, HTTPException, status
 from fastapi.security import OAuth2PasswordRequestForm
 from sqlalchemy.ext.asyncio import AsyncSession
 from sqlalchemy import select
-from datetime import datetime
+from datetime import datetime, timezone
 import logging
 
 from app.core.database import get_db
@@ -86,7 +86,7 @@ async def login(
         )
 
     # Update last login
-    user.last_login_at = datetime.utcnow()  # type: ignore[assignment]
+    user.last_login_at = datetime.now(timezone.utc)  # type: ignore[assignment]
     await db.commit()
 
     # Create tokens
@@ -133,7 +133,7 @@ async def google_oauth(
             user.oauth_provider = "google"  # type: ignore[assignment]
 
         # Update last login
-        user.last_login_at = datetime.utcnow()  # type: ignore[assignment]
+        user.last_login_at = datetime.now(timezone.utc)  # type: ignore[assignment]
 
         logger.info(f"Existing user logged in with Google: {user.email}")
     else:
@@ -145,7 +145,7 @@ async def google_oauth(
             oauth_provider="google",
             subscription_tier=SubscriptionTier.FREE,
             is_active=True,
-            last_login_at=datetime.utcnow(),
+            last_login_at=datetime.now(timezone.utc),
         )
         db.add(user)