feat(logger): add configurable log level resolver with environment defaults (#684)

[ravendevhub]

Summary

Resolves #684 by introducing a dedicated log level resolver module (listener/src/utils/log-level-resolver.ts) that validates LOG_LEVEL inputs against supported severities (debug, info, warn, error, silent), enforces production defaults (info), and handles invalid entries with deterministic fallback warnings.

Changes

1. Log Level Resolver (listener/src/utils/log-level-resolver.ts):
   • Resolves log level from environment variables with case-insensitivity and whitespace trimming.
   • Enforces default levels: info in production and debug in development.
   • Replaces invalid configurations with safe defaults accompanied by clear warnings.
2. Automated Unit Tests (listener/src/utils/log-level-resolver.test.ts):
   • Tests casing normalization, production/development defaults, and invalid fallback handling.
3. Documentation (docs/CONFIGURABLE_LOG_LEVEL.md):
   • Documents log levels, severity hierarchy, and fallback rules.

Acceptance Criteria

• Supported log levels are documented.
• Configuration can be supplied through the existing configuration mechanism.
• Invalid log levels are rejected.
• Production defaults remain appropriate.