feat: Week 3 Phase 1 - WAL Archiving & PITR Setup

## WAL Archiving Implementation (Phase 1/5)

### Core Components Created
- ✅ internal/wal/archiver.go (280 lines)
  - WAL file archiving with timeline/segment parsing
  - Archive statistics and cleanup
  - Compression/encryption scaffolding (TODO)

- ✅ internal/wal/pitr_config.go (360 lines)
  - PostgreSQL configuration management
  - auto-detects postgresql.conf location
  - Backs up config before modifications
  - Recovery configuration for PG 12+ and legacy

- ✅ cmd/pitr.go (350 lines)
  - pitr enable/disable/status commands
  - wal archive/list/cleanup commands
  - Integrated with existing CLI

### Features Implemented
**WAL Archiving:**
- ParseWALFileName: Extract timeline + segment from WAL files
- ArchiveWALFile: Copy WAL to archive directory
- ListArchivedWALFiles: View all archived WAL segments
- CleanupOldWALFiles: Retention-based cleanup
- GetArchiveStats: Statistics (total size, file count, date range)

**PITR Configuration:**
- EnablePITR: Auto-configure postgresql.conf for PITR
  - Sets wal_level=replica, archive_mode=on
  - Configures archive_command to call dbbackup
  - Creates WAL archive directory
- DisablePITR: Turn off WAL archiving
- GetCurrentPITRConfig: Read current settings
- CreateRecoveryConf: Generate recovery config (PG 12+ & legacy)

**CLI Commands:**
```bash
# Enable PITR
dbbackup pitr enable --archive-dir /backups/wal_archive

# Check PITR status
dbbackup pitr status

# Archive WAL file (called by PostgreSQL)
dbbackup wal archive <path> <filename> --archive-dir /backups/wal

# List WAL archives
dbbackup wal list --archive-dir /backups/wal_archive

# Cleanup old WAL files
dbbackup wal cleanup --archive-dir /backups/wal_archive --retention-days 7
```

### Architecture
- Modular design: Separate archiver and PITR manager
- PostgreSQL version detection (12+ vs legacy)
- Automatic config file discovery
- Safe config modifications with backups

### Next Steps (Phase 2)
- [ ] Compression support (gzip)
- [ ] Encryption support (AES-256-GCM)
- [ ] Continuous WAL monitoring
- [ ] Timeline management
- [ ] Point-in-time restore command

Time: ~1.5h (3h estimated for Phase 1)

internal/config/config.go

@@ -76,6 +76,12 @@ type Config struct {
 	AllowRoot      bool // Allow running as root/Administrator
 	CheckResources bool // Check resource limits before operations
 
+	// PITR (Point-in-Time Recovery) options
+	PITREnabled    bool   // Enable WAL archiving for PITR
+	WALArchiveDir  string // Directory to store WAL archives
+	WALCompression bool   // Compress WAL files
+	WALEncryption  bool   // Encrypt WAL files
+
 	// TUI automation options (for testing)
 	TUIAutoSelect   int    // Auto-select menu option (-1 = disabled)
 	TUIAutoDatabase string // Pre-fill database name