Feat/nm db retry logic

[sshenzha]

Key features added (db.py):

1. Thread-safe connection handling for concurrent access
2. Retry logic
3. Connection recovery on failures
4. Logging for connection attempts and failures

Key features added (main.py):

1. Logging system
2. Proper database connection cleanup on shutdown
3. Better error messages and status reporting

Key features added (sample.env)

1. Separated database-specific configuration
2. Provided complete configuration examples for both database types

[vvcb]

@shen1802 this looks good. Can you add a description in the PR about which github issue this solves and how?