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

internal/security/ratelimit.go

@@ -0,0 +1,176 @@
+package security
+
+import (
+	"fmt"
+	"sync"
+	"time"
+
+	"dbbackup/internal/logger"
+)
+
+// RateLimiter tracks connection attempts and enforces rate limiting
+type RateLimiter struct {
+	attempts      map[string]*attemptTracker
+	mu            sync.RWMutex
+	maxRetries    int
+	baseDelay     time.Duration
+	maxDelay      time.Duration
+	resetInterval time.Duration
+	log           logger.Logger
+}
+
+// attemptTracker tracks connection attempts for a specific host
+type attemptTracker struct {
+	count       int
+	lastAttempt time.Time
+	nextAllowed time.Time
+}
+
+// NewRateLimiter creates a new rate limiter for connection attempts
+func NewRateLimiter(maxRetries int, log logger.Logger) *RateLimiter {
+	return &RateLimiter{
+		attempts:      make(map[string]*attemptTracker),
+		maxRetries:    maxRetries,
+		baseDelay:     1 * time.Second,
+		maxDelay:      60 * time.Second,
+		resetInterval: 5 * time.Minute,
+		log:           log,
+	}
+}
+
+// CheckAndWait checks if connection is allowed and waits if rate limited
+// Returns error if max retries exceeded
+func (rl *RateLimiter) CheckAndWait(host string) error {
+	rl.mu.Lock()
+	defer rl.mu.Unlock()
+
+	now := time.Now()
+	tracker, exists := rl.attempts[host]
+
+	if !exists {
+		// First attempt, allow immediately
+		rl.attempts[host] = &attemptTracker{
+			count:       1,
+			lastAttempt: now,
+			nextAllowed: now,
+		}
+		return nil
+	}
+
+	// Reset counter if enough time has passed
+	if now.Sub(tracker.lastAttempt) > rl.resetInterval {
+		rl.log.Debug("Resetting rate limit counter", "host", host)
+		tracker.count = 1
+		tracker.lastAttempt = now
+		tracker.nextAllowed = now
+		return nil
+	}
+
+	// Check if max retries exceeded
+	if tracker.count >= rl.maxRetries {
+		return fmt.Errorf("max connection retries (%d) exceeded for host %s, try again in %v",
+			rl.maxRetries, host, rl.resetInterval)
+	}
+
+	// Calculate exponential backoff delay
+	delay := rl.calculateDelay(tracker.count)
+	tracker.nextAllowed = tracker.lastAttempt.Add(delay)
+
+	// Wait if necessary
+	if now.Before(tracker.nextAllowed) {
+		waitTime := tracker.nextAllowed.Sub(now)
+		rl.log.Info("Rate limiting connection attempt",
+			"host", host,
+			"attempt", tracker.count,
+			"wait_seconds", int(waitTime.Seconds()))
+
+		rl.mu.Unlock()
+		time.Sleep(waitTime)
+		rl.mu.Lock()
+	}
+
+	// Update tracker
+	tracker.count++
+	tracker.lastAttempt = time.Now()
+
+	return nil
+}
+
+// RecordSuccess resets the attempt counter for successful connections
+func (rl *RateLimiter) RecordSuccess(host string) {
+	rl.mu.Lock()
+	defer rl.mu.Unlock()
+
+	if tracker, exists := rl.attempts[host]; exists {
+		rl.log.Debug("Connection successful, resetting rate limit", "host", host)
+		tracker.count = 0
+		tracker.lastAttempt = time.Now()
+		tracker.nextAllowed = time.Now()
+	}
+}
+
+// RecordFailure increments the failure counter
+func (rl *RateLimiter) RecordFailure(host string) {
+	rl.mu.Lock()
+	defer rl.mu.Unlock()
+
+	now := time.Now()
+	tracker, exists := rl.attempts[host]
+
+	if !exists {
+		rl.attempts[host] = &attemptTracker{
+			count:       1,
+			lastAttempt: now,
+			nextAllowed: now.Add(rl.baseDelay),
+		}
+		return
+	}
+
+	tracker.count++
+	tracker.lastAttempt = now
+	tracker.nextAllowed = now.Add(rl.calculateDelay(tracker.count))
+
+	rl.log.Warn("Connection failed",
+		"host", host,
+		"attempt", tracker.count,
+		"max_retries", rl.maxRetries)
+}
+
+// calculateDelay calculates exponential backoff delay
+func (rl *RateLimiter) calculateDelay(attempt int) time.Duration {
+	// Exponential backoff: 1s, 2s, 4s, 8s, 16s, 32s, max 60s
+	delay := rl.baseDelay * time.Duration(1<<uint(attempt-1))
+	if delay > rl.maxDelay {
+		delay = rl.maxDelay
+	}
+	return delay
+}
+
+// GetStatus returns current rate limit status for a host
+func (rl *RateLimiter) GetStatus(host string) (attempts int, nextAllowed time.Time, isLimited bool) {
+	rl.mu.RLock()
+	defer rl.mu.RUnlock()
+
+	tracker, exists := rl.attempts[host]
+	if !exists {
+		return 0, time.Now(), false
+	}
+
+	now := time.Now()
+	isLimited = now.Before(tracker.nextAllowed)
+
+	return tracker.count, tracker.nextAllowed, isLimited
+}
+
+// Cleanup removes old entries from rate limiter
+func (rl *RateLimiter) Cleanup() {
+	rl.mu.Lock()
+	defer rl.mu.Unlock()
+
+	now := time.Now()
+	for host, tracker := range rl.attempts {
+		if now.Sub(tracker.lastAttempt) > rl.resetInterval*2 {
+			delete(rl.attempts, host)
+		}
+	}
+}