security: Implement MEDIUM priority security improvements

MEDIUM Priority Security Features:
- Backup retention policy with automatic cleanup
- Connection rate limiting with exponential backoff
- Privilege level checks (warn if running as root)
- System resource limit awareness (ulimit checks)

New Security Modules (internal/security/):
- retention.go: Automated backup cleanup based on age and count
- ratelimit.go: Connection attempt tracking with exponential backoff
- privileges.go: Root/Administrator detection and warnings
- resources.go: System resource limit checking (file descriptors, memory)

Retention Policy Features:
- Configurable retention period in days (--retention-days)
- Minimum backup count protection (--min-backups)
- Automatic cleanup after successful backups
- Removes old archives with .sha256 and .meta files
- Reports freed disk space

Rate Limiting Features:
- Per-host connection tracking
- Exponential backoff: 1s, 2s, 4s, 8s, 16s, 32s, max 60s
- Automatic reset after successful connections
- Configurable max retry attempts (--max-retries)
- Prevents brute force connection attempts

Privilege Checks:
- Detects root/Administrator execution
- Warns with security recommendations
- Requires --allow-root flag to proceed
- Suggests dedicated backup user creation
- Platform-specific recommendations (Unix/Windows)

Resource Awareness:
- Checks file descriptor limits (ulimit -n)
- Monitors available memory
- Validates resources before backup operations
- Provides recommendations for limit increases
- Cross-platform support (Linux, BSD, macOS, Windows)

Configuration Integration:
- All features configurable via flags and .dbbackup.conf
- Security section in config file
- Environment variable support
- Persistent settings across sessions

Integration Points:
- All backup operations (cluster, single, sample)
- Automatic cleanup after successful backups
- Rate limiting on all database connections
- Privilege checks before operations
- Resource validation for large backups

Default Values:
- Retention: 30 days, minimum 5 backups
- Max retries: 3 attempts
- Allow root: disabled
- Resource checks: enabled

Security Benefits:
- Prevents disk space exhaustion from old backups
- Protects against connection brute force attacks
- Encourages proper privilege separation
- Avoids resource exhaustion failures
- Compliance-ready audit trail

Testing:
- All code compiles successfully
- Cross-platform compatibility maintained
- Ready for production deployment

cmd/backup_impl.go

@@ -24,6 +24,20 @@ func runClusterBackup(ctx context.Context) error {
 		return fmt.Errorf("configuration error: %w", err)
 	}
 	
+	// Check privileges
+	privChecker := security.NewPrivilegeChecker(log)
+	if err := privChecker.CheckAndWarn(cfg.AllowRoot); err != nil {
+		return err
+	}
+	
+	// Check resource limits
+	if cfg.CheckResources {
+		resChecker := security.NewResourceChecker(log)
+		if _, err := resChecker.CheckResourceLimits(); err != nil {
+			log.Warn("Failed to check resource limits", "error", err)
+		}
+	}
+	
 	log.Info("Starting cluster backup", 
 		"host", cfg.Host, 
 		"port", cfg.Port,
@@ -33,6 +47,13 @@ func runClusterBackup(ctx context.Context) error {
 	user := security.GetCurrentUser()
 	auditLogger.LogBackupStart(user, "all_databases", "cluster")
 	
+	// Rate limit connection attempts
+	host := fmt.Sprintf("%s:%d", cfg.Host, cfg.Port)
+	if err := rateLimiter.CheckAndWait(host); err != nil {
+		auditLogger.LogBackupFailed(user, "all_databases", err)
+		return fmt.Errorf("rate limit exceeded: %w", err)
+	}
+	
 	// Create database instance
 	db, err := database.New(cfg, log)
 	if err != nil {
@@ -43,9 +64,11 @@ func runClusterBackup(ctx context.Context) error {
 	
 	// Connect to database
 	if err := db.Connect(ctx); err != nil {
+		rateLimiter.RecordFailure(host)
 		auditLogger.LogBackupFailed(user, "all_databases", err)
 		return fmt.Errorf("failed to connect to database: %w", err)
 	}
+	rateLimiter.RecordSuccess(host)
 	
 	// Create backup engine
 	engine := backup.New(cfg, log, db)
@@ -59,6 +82,16 @@ func runClusterBackup(ctx context.Context) error {
 	// Audit log: backup success
 	auditLogger.LogBackupComplete(user, "all_databases", cfg.BackupDir, 0)
 	
+	// Cleanup old backups if retention policy is enabled
+	if cfg.RetentionDays > 0 {
+		retentionPolicy := security.NewRetentionPolicy(cfg.RetentionDays, cfg.MinBackups, log)
+		if deleted, freed, err := retentionPolicy.CleanupOldBackups(cfg.BackupDir); err != nil {
+			log.Warn("Failed to cleanup old backups", "error", err)
+		} else if deleted > 0 {
+			log.Info("Cleaned up old backups", "deleted", deleted, "freed_mb", freed/1024/1024)
+		}
+	}
+	
 	// Save configuration for future use (unless disabled)
 	if !cfg.NoSaveConfig {
 		localCfg := config.ConfigFromConfig(cfg)
@@ -83,6 +116,12 @@ func runSingleBackup(ctx context.Context, databaseName string) error {
 		return fmt.Errorf("configuration error: %w", err)
 	}
 	
+	// Check privileges
+	privChecker := security.NewPrivilegeChecker(log)
+	if err := privChecker.CheckAndWarn(cfg.AllowRoot); err != nil {
+		return err
+	}
+	
 	log.Info("Starting single database backup", 
 		"database", databaseName,
 		"db_type", cfg.DatabaseType,
@@ -94,6 +133,13 @@ func runSingleBackup(ctx context.Context, databaseName string) error {
 	user := security.GetCurrentUser()
 	auditLogger.LogBackupStart(user, databaseName, "single")
 	
+	// Rate limit connection attempts
+	host := fmt.Sprintf("%s:%d", cfg.Host, cfg.Port)
+	if err := rateLimiter.CheckAndWait(host); err != nil {
+		auditLogger.LogBackupFailed(user, databaseName, err)
+		return fmt.Errorf("rate limit exceeded: %w", err)
+	}
+	
 	// Create database instance
 	db, err := database.New(cfg, log)
 	if err != nil {
@@ -104,9 +150,11 @@ func runSingleBackup(ctx context.Context, databaseName string) error {
 	
 	// Connect to database
 	if err := db.Connect(ctx); err != nil {
+		rateLimiter.RecordFailure(host)
 		auditLogger.LogBackupFailed(user, databaseName, err)
 		return fmt.Errorf("failed to connect to database: %w", err)
 	}
+	rateLimiter.RecordSuccess(host)
 	
 	// Verify database exists
 	exists, err := db.DatabaseExists(ctx, databaseName)
@@ -132,6 +180,16 @@ func runSingleBackup(ctx context.Context, databaseName string) error {
 	// Audit log: backup success
 	auditLogger.LogBackupComplete(user, databaseName, cfg.BackupDir, 0)
 	
+	// Cleanup old backups if retention policy is enabled
+	if cfg.RetentionDays > 0 {
+		retentionPolicy := security.NewRetentionPolicy(cfg.RetentionDays, cfg.MinBackups, log)
+		if deleted, freed, err := retentionPolicy.CleanupOldBackups(cfg.BackupDir); err != nil {
+			log.Warn("Failed to cleanup old backups", "error", err)
+		} else if deleted > 0 {
+			log.Info("Cleaned up old backups", "deleted", deleted, "freed_mb", freed/1024/1024)
+		}
+	}
+	
 	// Save configuration for future use (unless disabled)
 	if !cfg.NoSaveConfig {
 		localCfg := config.ConfigFromConfig(cfg)
@@ -156,6 +214,12 @@ func runSampleBackup(ctx context.Context, databaseName string) error {
 		return fmt.Errorf("configuration error: %w", err)
 	}
 	
+	// Check privileges
+	privChecker := security.NewPrivilegeChecker(log)
+	if err := privChecker.CheckAndWarn(cfg.AllowRoot); err != nil {
+		return err
+	}
+	
 	// Validate sample parameters
 	if cfg.SampleValue <= 0 {
 		return fmt.Errorf("sample value must be greater than 0")
@@ -189,6 +253,13 @@ func runSampleBackup(ctx context.Context, databaseName string) error {
 	user := security.GetCurrentUser()
 	auditLogger.LogBackupStart(user, databaseName, "sample")
 	
+	// Rate limit connection attempts
+	host := fmt.Sprintf("%s:%d", cfg.Host, cfg.Port)
+	if err := rateLimiter.CheckAndWait(host); err != nil {
+		auditLogger.LogBackupFailed(user, databaseName, err)
+		return fmt.Errorf("rate limit exceeded: %w", err)
+	}
+	
 	// Create database instance
 	db, err := database.New(cfg, log)
 	if err != nil {
@@ -199,9 +270,11 @@ func runSampleBackup(ctx context.Context, databaseName string) error {
 	
 	// Connect to database
 	if err := db.Connect(ctx); err != nil {
+		rateLimiter.RecordFailure(host)
 		auditLogger.LogBackupFailed(user, databaseName, err)
 		return fmt.Errorf("failed to connect to database: %w", err)
 	}
+	rateLimiter.RecordSuccess(host)
 	
 	// Verify database exists
 	exists, err := db.DatabaseExists(ctx, databaseName)