v3.40.0: Restore diagnostics and error reporting

Features:
- restore diagnose command for backup file analysis
- Deep COPY block verification for truncated dump detection
- PGDMP signature and gzip integrity validation
- Detailed error reports with --save-debug-log flag
- Ring buffer stderr capture (prevents OOM on 2M+ errors)
- Error classification with actionable recommendations

TUI Enhancements:
- Automatic dump validity safety check before restore
- Press 'd' in archive browser to diagnose backups
- Press 'd' in restore preview for debug log toggle
- Debug logs saved to /tmp on failure when enabled

Documentation:
- Updated README with diagnose command and examples
- Updated CHANGELOG with full feature list
- Updated restore preview screenshots

internal/engine/snapshot_engine.go

@@ -223,7 +223,7 @@ func (e *SnapshotEngine) Backup(ctx context.Context, opts *BackupOptions) (*Back
 
 	// Save metadata
 	meta := &metadata.BackupMetadata{
-		Version:      "3.2.0",
+		Version:      "3.40.0",
 		Timestamp:    startTime,
 		Database:     opts.Database,
 		DatabaseType: "mysql",