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/models/tree.py

@@ -1,5 +1,5 @@
 import uuid
-from datetime import datetime
+from datetime import datetime, timezone
 from typing import Optional, Any
 from sqlalchemy import String, Text, DateTime, ForeignKey, Boolean, Integer, Index
 from sqlalchemy.orm import Mapped, mapped_column, relationship
@@ -33,13 +33,13 @@ class Tree(Base):
     is_active: Mapped[bool] = mapped_column(Boolean, default=True)
     version: Mapped[int] = mapped_column(Integer, default=1)
     created_at: Mapped[datetime] = mapped_column(
-        DateTime,
-        default=datetime.utcnow
+        DateTime(timezone=True),
+        default=lambda: datetime.now(timezone.utc)
     )
     updated_at: Mapped[datetime] = mapped_column(
-        DateTime,
-        default=datetime.utcnow,
-        onupdate=datetime.utcnow
+        DateTime(timezone=True),
+        default=lambda: datetime.now(timezone.utc),
+        onupdate=lambda: datetime.now(timezone.utc)
     )
     usage_count: Mapped[int] = mapped_column(Integer, default=0)