chore: repo cleanup + docs for admin API and DB schema (v1.2.8)

- remove stale docs/admin-frontend-spec.md (old Express/EJS admin), unused templates/ (SmartAdmin copy), dead scripts/sync-agents.cjs, committed dev.pid
- add docs/API.md (admin REST API reference) and docs/DATABASE.md (DB schema)
- refresh .env.example: drop stale ADMIN_PORT/SHOP_CONTAINER, add ADMIN_URL, HEALTH_PORT, DEFAULT_LANGUAGE, CHATBOT_API_*
- add npm run lint so Gitea workflows pass
- rebrand web-testing suite from APAW to telegram-shop

.gitignore

@@ -21,11 +21,8 @@ db/*.pre-merge-*
 # Root-level throwaway scripts
 /*.mjs
 
-/*.mjs
-
 # Production backups (contain secrets + DB snapshots — never commit)
 production-backup/
-screenshot-dash.cjs
 
 # Kilo agent configuration (managed locally, not committed)
 .kilo/
@@ -33,3 +30,8 @@ kilo-meta.json
 kilo.jsonc
 AGENTS.md
 admin-next/.env
+
+# Admin dev artifacts
+admin-next/.zscripts/*.pid
+admin-next/.next/
+admin-next/node_modules/