refactor(logging): reduce download log spam and set INFO level

- Pass app logger to yt-dlp so internal [download] progress lines
  are routed through the INFO-level logger instead of stdout.
- Throttle download_progress_handler debug logging to avoid
  flooding logs on every fragment tick.
- Switch key provider lifecycle messages to INFO (start/complete)
  while keeping verbose details at DEBUG.
- Set debug_enabled=False in development config so dev mode
  does not emit extra debug noise.
- Update config docstring example from DEBUG to INFO.

src/server/config/__init__.py

@@ -39,7 +39,7 @@ def get_settings() -> Union[DevelopmentSettings, ProductionSettings]:
     Example:
         >>> settings = get_settings()
         >>> print(settings.log_level)
-        DEBUG
+        INFO
     """
     if ENVIRONMENT in {"development", "testing"}:
         return get_development_settings()