M19: deriveStackName cross-references deployed stacks (fix DB-container misattribution)

deriveStackName pure-suffix-stripped on '-' (postgres/db/mariadb/.../cache), so a
stack whose slug ENDS in a role token (e.g. 'my-cache') was misattributed (stripped
to 'my') — filing its DB dump under the wrong/nonexistent stack. Now threads the set
of deployed stack names (m.knownStackNames() <- ListDeployedStacks) into
DiscoverDatabases and cross-references: candidate suffix-strip if known, else the
container name if it IS a known stack, else longest known stack that is a prefix
(handles <stack>_postgres / <stack>-1), else legacy strip. nil/empty known = legacy
behaviour (appexport passes nil). Table test incl. the my-cache case (fails pre-fix).

controller/internal/backup/restore_db.go

@@ -41,7 +41,7 @@ func (m *Manager) reimportDBDumps(ctx context.Context, stackName, nsRoot string)
 	discover := m.discoverDBs
 	if discover == nil {
 		discover = func(ctx context.Context) ([]DiscoveredDB, error) {
-			return DiscoverDatabases(ctx, m.logger, m.isDebug())
+			return DiscoverDatabases(ctx, m.logger, m.isDebug(), m.knownStackNames())
 		}
 	}
 	imp := m.importDBDump