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/main.py

@@ -2,6 +2,8 @@
 Main FastAPI application.
 """
 
+from contextlib import asynccontextmanager
+from collections.abc import AsyncIterator
 from fastapi import FastAPI
 from fastapi.middleware.cors import CORSMiddleware
 import logging
@@ -19,6 +21,18 @@ logging.basicConfig(
 logger = logging.getLogger(__name__)
 
 
+@asynccontextmanager
+async def lifespan(app: FastAPI) -> AsyncIterator[None]:
+    """Application lifespan handler for startup and shutdown events."""
+    # Startup
+    logger.info(f"Starting {settings.APP_NAME} v{settings.APP_VERSION}")
+    logger.info(f"Debug mode: {settings.DEBUG}")
+    logger.info("API documentation: /api/docs")
+    yield
+    # Shutdown
+    logger.info("Shutting down application")
+
+
 def create_application() -> FastAPI:
     """Create and configure FastAPI application"""
 
@@ -29,6 +43,7 @@ def create_application() -> FastAPI:
         docs_url="/api/docs",
         redoc_url="/api/redoc",
         openapi_url="/api/openapi.json",
+        lifespan=lifespan,
     )
 
     # Security middleware (add before CORS)
@@ -61,18 +76,6 @@ def create_application() -> FastAPI:
         """Health check endpoint for container orchestration"""
         return {"status": "healthy"}
 
-    @app.on_event("startup")
-    async def startup_event():
-        """Run on application startup"""
-        logger.info(f"Starting {settings.APP_NAME} v{settings.APP_VERSION}")
-        logger.info(f"Debug mode: {settings.DEBUG}")
-        logger.info("API documentation: /api/docs")
-
-    @app.on_event("shutdown")
-    async def shutdown_event():
-        """Run on application shutdown"""
-        logger.info("Shutting down application")
-
     return app