refactor(infra): remove citycorners + skilltree NestJS backends, clean up CI/CD

Both apps migrated to local-first (mana-sync handles CRUD).

- Delete apps/citycorners/apps/backend/ (37 files)
- Delete apps/skilltree/apps/backend/ (32 files)
- Remove from CI build jobs, change detection, summary
- Remove from package.json scripts (replaced with sync-based dev commands)
- Remove from setup-databases.sh push_schema calls
- Remove from generate-env.mjs backend env generation
- Remove from ensure-containers-running.sh

Total: 6 NestJS backends removed across all sessions (Zitare, Clock,
Presi, Photos, CityCorners, SkillTree). ~12,000 lines of boilerplate
eliminated.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

scripts/setup-databases.sh

@@ -192,7 +192,7 @@ setup_service() {
             ;;
         skilltree)
             create_db_if_not_exists "skilltree"
-            push_schema "@skilltree/backend" "skilltree"
+            # Schema managed by mana-sync (backend removed)
             ;;
         mukke)
             create_db_if_not_exists "mukke"
@@ -208,7 +208,7 @@ setup_service() {
             ;;
         citycorners)
             create_db_if_not_exists "citycorners"
-            push_schema "@citycorners/backend" "citycorners"
+            # Schema managed by mana-sync (backend removed)
             ;;
         *)
             echo -e "${RED}Unknown service: $service${NC}"