Add automated backup system with SQLite VACUUM

- Implement backup package with SQLite VACUUM INTO for safe backups
- Add scheduler with configurable interval (default: 24h)
- Add retention policy with automatic cleanup (default: 7 days)
- Add optional gzip compression (~10x size reduction)
- Add comprehensive tests for backup creation and cleanup
- Integrate backup scheduler in main.go with graceful shutdown
- Add backup configuration variables (BACKUP_ENABLED, BACKUP_INTERVAL, BACKUP_RETENTION_DAYS, BACKUP_PATH, BACKUP_COMPRESS)
- Backups run automatically in background goroutine
- Coverage: backup package 52.7%

internal/infrastructure/backup/retention.go

@@ -0,0 +1,80 @@
+package backup
+
+import (
+	"os"
+	"path/filepath"
+	"strings"
+	"time"
+
+	"apocapoc-api/internal/infrastructure/logger"
+)
+
+func CleanOldBackups(config Config) error {
+	if !config.Enabled {
+		return nil
+	}
+
+	if config.RetentionDays <= 0 {
+		return nil
+	}
+
+	cutoffTime := time.Now().AddDate(0, 0, -config.RetentionDays)
+
+	files, err := os.ReadDir(config.Path)
+	if err != nil {
+		if os.IsNotExist(err) {
+			return nil
+		}
+		return err
+	}
+
+	deletedCount := 0
+	for _, file := range files {
+		if file.IsDir() {
+			continue
+		}
+
+		if !strings.HasPrefix(file.Name(), "apocapoc_") {
+			continue
+		}
+
+		if !strings.HasSuffix(file.Name(), ".db") && !strings.HasSuffix(file.Name(), ".db.gz") {
+			continue
+		}
+
+		filePath := filepath.Join(config.Path, file.Name())
+		info, err := os.Stat(filePath)
+		if err != nil {
+			logger.Warn().
+				Err(err).
+				Str("file", filePath).
+				Msg("Failed to stat backup file")
+			continue
+		}
+
+		if info.ModTime().Before(cutoffTime) {
+			if err := os.Remove(filePath); err != nil {
+				logger.Warn().
+					Err(err).
+					Str("file", filePath).
+					Msg("Failed to delete old backup")
+				continue
+			}
+
+			logger.Info().
+				Str("file", file.Name()).
+				Time("mod_time", info.ModTime()).
+				Msg("Deleted old backup")
+			deletedCount++
+		}
+	}
+
+	if deletedCount > 0 {
+		logger.Info().
+			Int("deleted_count", deletedCount).
+			Int("retention_days", config.RetentionDays).
+			Msg("Backup cleanup completed")
+	}
+
+	return nil
+}