Production Stability Upgrade

Added:
- Global error handlers: 404, 403, 500, database error, unexpected error.
- Friendly error pages in templates/errors/.
- Server-side error logging to logs/error.log.
- Rotating log file to prevent unlimited log growth.
- debug=False for safer production use.
- Stable SECRET_KEY via environment variable support.

Important:
Before hosting, set SECRET_KEY environment variable to a strong random value.
Check logs/error.log when any page fails.