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/appbackup_bridge.go

@@ -47,8 +47,8 @@ const FelhomDataDir = appbackup.FelhomDataDir
 
 // --- function forwarders (dbdump) ---
 
-func DiscoverDatabases(ctx context.Context, logger *log.Logger, debug bool) ([]DiscoveredDB, error) {
-	return appbackup.DiscoverDatabases(ctx, logger, debug)
+func DiscoverDatabases(ctx context.Context, logger *log.Logger, debug bool, knownStacks []string) ([]DiscoveredDB, error) {
+	return appbackup.DiscoverDatabases(ctx, logger, debug, knownStacks)
 }
 
 func DumpAll(ctx context.Context, dbs []DiscoveredDB, dumpDir string, logger *log.Logger, debug bool) []DumpResult {