Add production logging, datetime fixes, and session tests

DateTime Timezone Handling (Critical Bug Fix):
- Updated all models to use DateTime(timezone=True) for PostgreSQL
- Changed datetime defaults to lambda: datetime.now(timezone.utc)
- Fixed mixing of timezone-aware and timezone-naive datetime objects
- Resolved Internal Server Errors in session completion endpoint
- Affected models: User, Team, Tree, Session, Attachment

Production Logging System:
- Created logging_config.py with structured logging setup
- Added log rotation (10MB files, 10 backups) for production
- Implemented RequestLoggingMiddleware with correlation IDs
- Added ErrorLoggingMiddleware for comprehensive error tracking
- Integrated logging into main.py application startup
- Supports dev/prod modes with appropriate log levels

Integration Tests - Session Workflow:
- Created test_sessions.py with 12 comprehensive tests
- Session lifecycle: create, update, complete
- Session export in multiple formats (markdown, text, HTML)
- Error handling and authorization checks
- Added pytest.ini with coverage configuration
- Added requirements-dev.txt with pytest dependencies

Following 2026 FastAPI best practices for timezone handling and structured logging.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

backend/app/main.py

@@ -1,21 +1,30 @@
+import logging
 from contextlib import asynccontextmanager
 from fastapi import FastAPI
 from fastapi.middleware.cors import CORSMiddleware
 
 from app.core.config import settings
 from app.core.database import init_db
+from app.core.logging_config import setup_logging
+from app.core.middleware import RequestLoggingMiddleware, ErrorLoggingMiddleware
 from app.api.router import api_router
 
+# Initialize logging configuration
+setup_logging()
+logger = logging.getLogger(__name__)
+
 
 @asynccontextmanager
 async def lifespan(app: FastAPI):
     """Application lifespan handler."""
     # Startup
+    logger.info("Starting Apoklisis API server...")
+    logger.info(f"Environment: {'Development' if settings.DEBUG else 'Production'}")
     # Note: In production, use Alembic migrations instead of init_db
     # await init_db()
     yield
     # Shutdown
-    pass
+    logger.info("Shutting down Apoklisis API server...")
 
 
 app = FastAPI(
@@ -28,6 +37,10 @@ app = FastAPI(
     lifespan=lifespan
 )
 
+# Add logging middleware (BEFORE CORS to log all requests)
+app.add_middleware(ErrorLoggingMiddleware)
+app.add_middleware(RequestLoggingMiddleware)
+
 # Configure CORS
 app.add_middleware(
     CORSMiddleware,