feat: base volume path setting

Add global base_volume_path to settings. Relative volume source
paths are automatically prepended with the base path at deploy
time. Absolute paths are used as-is. Configurable in Settings >
General.

internal/store/store.go

@@ -71,6 +71,8 @@ func (s *Store) runMigrations() error {
 	migrations := []string{
 		// Add owner column to registries (2026-03-28).
 		`ALTER TABLE registries ADD COLUMN owner TEXT NOT NULL DEFAULT ''`,
+		// Add base_volume_path to settings (2026-03-28).
+		`ALTER TABLE settings ADD COLUMN base_volume_path TEXT NOT NULL DEFAULT ''`,
 	}
 
 	for _, m := range migrations {
@@ -131,6 +133,7 @@ CREATE TABLE IF NOT EXISTS settings (
 	npm_password        TEXT NOT NULL DEFAULT '',
 	webhook_secret      TEXT NOT NULL DEFAULT '',
 	polling_interval    TEXT NOT NULL DEFAULT '5m',
+	base_volume_path   TEXT NOT NULL DEFAULT '',
 	updated_at          TEXT NOT NULL DEFAULT (datetime('now'))
 );