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/pytest.ini

@@ -0,0 +1,38 @@
+[pytest]
+# Pytest configuration for Apoklisis backend tests
+
+# Python path - add current directory so 'app' module can be imported
+pythonpath = .
+
+# Asyncio configuration
+asyncio_mode = auto
+
+# Test discovery patterns
+python_files = test_*.py
+python_classes = Test*
+python_functions = test_*
+
+# Output options
+addopts =
+    -v
+    --strict-markers
+    --tb=short
+    --cov=app
+    --cov-report=term-missing
+    --cov-report=html
+
+# Test markers
+markers =
+    asyncio: marks tests as async
+    slow: marks tests as slow (deselect with '-m "not slow"')
+    integration: marks tests as integration tests
+    unit: marks tests as unit tests
+
+# Ignore paths
+testpaths = tests
+
+# Warnings
+filterwarnings =
+    error
+    ignore::DeprecationWarning
+    ignore::PendingDeprecationWarning