feat: add mergerfs mover and snapraid integration

New 'Almacenamiento' page with:
- Auto-detection of rsync, mergerfs, snapraid binaries and mergerfs mount
- Configurable pool settings (source/dest, macOS cleanup, rsync flags)
- Mergerfs mover with dry-run preview and live SSE output streaming
- SnapRAID diff/sync/scrub/check with live SSE output
- Async job system (1 concurrent job) with SSE streaming
- Job history table

Backend:
- internal/storage/ package with capabilities, mergerfs, snapraid, jobs
- storage_config and storage_jobs DB tables (migration 0008)
- GET/PUT /api/storage/config, GET /api/storage/capabilities
- POST/GET /api/storage/jobs, GET /api/storage/jobs/{id}/stream
- Storage operations disabled when NASCTL_EXEC_SYSTEM=false

Closes #new-feature

internal/db/db.go

@@ -91,6 +91,11 @@ func (d *DB) Migrate() error {
 				return fmt.Errorf("fix zero fsids: %w", err)
 			}
 		}
+		if name == "0008_storage.sql" {
+			if err := d.EnsureStorageConfig(); err != nil {
+				return fmt.Errorf("ensure storage config: %w", err)
+			}
+		}
 	}
 	return nil
 }