feat: Phase 3A - Incremental backup scaffolding (types, interfaces, metadata)

Added foundational types for PostgreSQL incremental backups:

Types & Interfaces (internal/backup/incremental.go):
- BackupType enum: full vs incremental
- IncrementalMetadata struct with base backup reference
- ChangedFile struct for tracking modifications
- BackupChainResolver interface for restore chain logic
- IncrementalBackupEngine interface

PostgreSQL Implementation (internal/backup/incremental_postgres.go):
- PostgresIncrementalEngine for file-level incrementals
- FindChangedFiles() - mtime-based change detection
- shouldSkipFile() - exclude temp/lock/socket files
- loadBackupInfo() - read base backup metadata
- Stubs for CreateIncrementalBackup() and RestoreIncremental()

Metadata Extension (internal/metadata/metadata.go):
- Added IncrementalMetadata to BackupMetadata
- Fields: base_backup_id, backup_chain, incremental_files
- Tracks parent backup and restore dependencies

Next Steps:
- Add --backup-type incremental flag to CLI
- Implement backup chain resolution
- Write integration tests

Status: SCAFFOLDING ONLY - not functional yet

internal/backup/incremental.go

@@ -0,0 +1,108 @@
+package backup
+
+import (
+	"context"
+	"time"
+)
+
+// BackupType represents the type of backup
+type BackupType string
+
+const (
+	BackupTypeFull        BackupType = "full"        // Complete backup of all data
+	BackupTypeIncremental BackupType = "incremental" // Only changed files since base backup
+)
+
+// IncrementalMetadata contains metadata for incremental backups
+type IncrementalMetadata struct {
+	// BaseBackupID is the SHA-256 checksum of the base backup this incremental depends on
+	BaseBackupID string `json:"base_backup_id"`
+	
+	// BaseBackupPath is the filename of the base backup (e.g., "mydb_20250126_120000.tar.gz")
+	BaseBackupPath string `json:"base_backup_path"`
+	
+	// BaseBackupTimestamp is when the base backup was created
+	BaseBackupTimestamp time.Time `json:"base_backup_timestamp"`
+	
+	// IncrementalFiles is the number of changed files included in this backup
+	IncrementalFiles int `json:"incremental_files"`
+	
+	// TotalSize is the total size of changed files (bytes)
+	TotalSize int64 `json:"total_size"`
+	
+	// BackupChain is the list of all backups needed for restore (base + incrementals)
+	// Ordered from oldest to newest: [base, incr1, incr2, ...]
+	BackupChain []string `json:"backup_chain"`
+}
+
+// ChangedFile represents a file that changed since the base backup
+type ChangedFile struct {
+	// RelativePath is the path relative to PostgreSQL data directory
+	RelativePath string
+	
+	// AbsolutePath is the full filesystem path
+	AbsolutePath string
+	
+	// Size is the file size in bytes
+	Size int64
+	
+	// ModTime is the last modification time
+	ModTime time.Time
+	
+	// Checksum is the SHA-256 hash of the file content (optional)
+	Checksum string
+}
+
+// IncrementalBackupConfig holds configuration for incremental backups
+type IncrementalBackupConfig struct {
+	// BaseBackupPath is the path to the base backup archive
+	BaseBackupPath string
+	
+	// DataDirectory is the PostgreSQL data directory to scan
+	DataDirectory string
+	
+	// IncludeWAL determines if WAL files should be included
+	IncludeWAL bool
+	
+	// CompressionLevel for the incremental archive (0-9)
+	CompressionLevel int
+}
+
+// BackupChainResolver resolves the chain of backups needed for restore
+type BackupChainResolver interface {
+	// FindBaseBackup locates the base backup for an incremental backup
+	FindBaseBackup(ctx context.Context, incrementalBackupID string) (*BackupInfo, error)
+	
+	// ResolveChain returns the complete chain of backups needed for restore
+	// Returned in order: [base, incr1, incr2, ..., target]
+	ResolveChain(ctx context.Context, targetBackupID string) ([]*BackupInfo, error)
+	
+	// ValidateChain verifies all backups in the chain exist and are valid
+	ValidateChain(ctx context.Context, chain []*BackupInfo) error
+}
+
+// IncrementalBackupEngine handles incremental backup operations
+type IncrementalBackupEngine interface {
+	// FindChangedFiles identifies files changed since the base backup
+	FindChangedFiles(ctx context.Context, config *IncrementalBackupConfig) ([]ChangedFile, error)
+	
+	// CreateIncrementalBackup creates a new incremental backup
+	CreateIncrementalBackup(ctx context.Context, config *IncrementalBackupConfig, changedFiles []ChangedFile) error
+	
+	// RestoreIncremental restores an incremental backup on top of a base backup
+	RestoreIncremental(ctx context.Context, baseBackupPath, incrementalPath, targetDir string) error
+}
+
+// BackupInfo extends the existing Info struct with incremental metadata
+// This will be integrated into the existing backup.Info struct
+type BackupInfo struct {
+	// Existing fields from backup.Info...
+	Database  string    `json:"database"`
+	Timestamp time.Time `json:"timestamp"`
+	Size      int64     `json:"size"`
+	Checksum  string    `json:"checksum"`
+	
+	// New fields for incremental support
+	BackupType BackupType           `json:"backup_type"`           // "full" or "incremental"
+	Incremental *IncrementalMetadata `json:"incremental,omitempty"` // Only present for incremental backups
+}