Enhance DatabaseManager with logging, validation, and cleanup

- Added detailed logging for database initialization and connection handling.
- Implemented configuration validation and sanity checks during initialization.
- Introduced `close()` method to properly release database resources.
- Updated `MediaManagerApplication` to invoke `DatabaseManager.close()` on shutdown.
- Updated `.gitignore` to exclude sensitive configuration files while allowing example files.

src/main/java/com/mediamanager/MediaManagerApplication.java

@@ -33,7 +33,9 @@ public class MediaManagerApplication {
             // Keep application running
             Runtime.getRuntime().addShutdownHook(new Thread(() -> {
                 logger.info("Shutting down MediaManager Core...");
-                // TODO: Cleanup resources
+                if (databaseManager != null) {
+                    databaseManager.close();
+                }
             }));
 
         } catch (Exception e) {