feat: add dry-run mode, GFS retention policies, and notifications

- Add --dry-run/-n flag for backup commands with comprehensive preflight checks
  - Database connectivity validation
  - Required tools availability check
  - Storage target and permissions verification
  - Backup size estimation
  - Encryption and cloud storage configuration validation

- Implement GFS (Grandfather-Father-Son) retention policies
  - Daily/Weekly/Monthly/Yearly tier classification
  - Configurable retention counts per tier
  - Custom weekly day and monthly day settings
  - ISO week handling for proper week boundaries

- Add notification system with SMTP and webhook support
  - SMTP email notifications with TLS/STARTTLS
  - Webhook HTTP notifications with HMAC-SHA256 signing
  - Slack-compatible webhook payload format
  - Event types: backup/restore started/completed/failed, cleanup, verify, PITR
  - Configurable severity levels and retry logic

- Update README.md with documentation for all new features

cmd/backup.go

@@ -48,6 +48,7 @@ var (
 	encryptBackupFlag bool
 	encryptionKeyFile string
 	encryptionKeyEnv  string
+	backupDryRun      bool
 )
 
 var singleCmd = &cobra.Command{
@@ -123,6 +124,11 @@ func init() {
 		cmd.Flags().StringVar(&encryptionKeyEnv, "encryption-key-env", "DBBACKUP_ENCRYPTION_KEY", "Environment variable containing encryption key/passphrase")
 	}
 
+	// Dry-run flag for all backup commands
+	for _, cmd := range []*cobra.Command{clusterCmd, singleCmd, sampleCmd} {
+		cmd.Flags().BoolVarP(&backupDryRun, "dry-run", "n", false, "Validate configuration without executing backup")
+	}
+
 	// Cloud storage flags for all backup commands
 	for _, cmd := range []*cobra.Command{clusterCmd, singleCmd, sampleCmd} {
 		cmd.Flags().String("cloud", "", "Cloud storage URI (e.g., s3://bucket/path) - takes precedence over individual flags")