Add reliability improvements and config persistence feature

- Implement context cleanup with sync.Once and io.Closer interface
- Add regex-based error classification for robust error handling
- Create ProcessManager with thread-safe process tracking
- Add disk space caching with 30s TTL for performance
- Implement metrics collection with structured logging
- Add config persistence (.dbbackup.conf) for directory-local settings
- Auto-save/auto-load configuration with --no-config and --no-save-config flags
- Successfully tested with 42GB d7030 database (35K large objects, 36min backup)
- All cross-platform builds working (9/10 platforms)

internal/config/config.go

@@ -49,6 +49,10 @@ type Config struct {
 	Debug        bool
 	LogLevel     string
 	LogFormat    string
+	
+	// Config persistence
+	NoSaveConfig bool
+	NoLoadConfig bool
 	OutputLength int
 
 	// Single database backup/restore