docs: update all markdown files, switch remote to Gitea

- Update root README.md: remove Leptos/WASM/SQLite, document Next.js
- Update frontend README.md: replace create-next-app boilerplate
- Update frontend AGENTS.md: project-specific overview
- Update CLAUDE.md: remove all auth references
- Update backend ARCHITECTURE.md: remove adminAuth from middleware list
- Switch git remote from gitlab to gitea (origin → gitea)

services/backend/ARCHITECTURE.md

@@ -15,7 +15,7 @@ services/backend/
 │   │   ├── logger/
 │   │   │   └── index.ts                 # Pino logger with child context support
 │   │   ├── middlewares/
-│   │   │   └── index.ts                 # Express middleware (errorHandler, asyncHandler, adminAuth)
+│   │   │   └── index.ts                 # Express middleware (errorHandler, asyncHandler, rateLimit)
 │   │   └── utils/                       # Utility functions (placeholder)
 │   │
 │   ├── modules/                         # Feature modules (Modular MVC pattern)