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/providers.py

@@ -1,6 +1,6 @@
 """Provider presets and Gmail credential management endpoints"""
 
-from datetime import datetime
+from datetime import datetime, timezone
 from typing import List
 from fastapi import APIRouter, Depends, HTTPException, status
 from sqlalchemy.ext.asyncio import AsyncSession
@@ -205,7 +205,7 @@ async def save_gmail_credential(
         existing.encrypted_access_token = encrypted_access  # type: ignore[assignment]
         existing.encrypted_refresh_token = encrypted_refresh  # type: ignore[assignment]
         existing.is_valid = True  # type: ignore[assignment]
-        existing.last_verified_at = datetime.utcnow()  # type: ignore[assignment]
+        existing.last_verified_at = datetime.now(timezone.utc)  # type: ignore[assignment]
         await db.commit()
         await db.refresh(existing)
         return existing
@@ -217,7 +217,7 @@ async def save_gmail_credential(
             encrypted_access_token=encrypted_access,
             encrypted_refresh_token=encrypted_refresh,
             is_valid=True,
-            last_verified_at=datetime.utcnow(),
+            last_verified_at=datetime.now(timezone.utc),
         )
         db.add(credential)
         await db.commit()