Add exclude patterns, snapshot prune, and other improvements

- Implement exclude patterns with anchored pattern support:
  - Patterns starting with / only match from root of source dir
  - Unanchored patterns match anywhere in path
  - Support for glob patterns (*.log, .*, **/*.pack)
  - Directory patterns skip entire subtrees
  - Add gobwas/glob dependency for pattern matching
  - Add 16 comprehensive tests for exclude functionality

- Add snapshot prune command to clean orphaned data:
  - Removes incomplete snapshots from database
  - Cleans orphaned files, chunks, and blobs
  - Runs automatically at backup start for consistency

- Add snapshot remove command for deleting snapshots

- Add VAULTIK_AGE_SECRET_KEY environment variable support

- Fix duplicate fx module provider in restore command

- Change snapshot ID format to hostname_YYYY-MM-DDTHH:MM:SSZ

internal/config/config.go

@@ -146,6 +146,11 @@ func Load(path string) (*Config, error) {
 		cfg.IndexPath = expandTilde(envIndexPath)
 	}
 
+	// Check for environment variable override for AgeSecretKey
+	if envAgeSecretKey := os.Getenv("VAULTIK_AGE_SECRET_KEY"); envAgeSecretKey != "" {
+		cfg.AgeSecretKey = envAgeSecretKey
+	}
+
 	// Get hostname if not set
 	if cfg.Hostname == "" {
 		hostname, err := os.Hostname()