Remove deprecated agent and prompt files; add new architecture, contract, and feature documentation for Reader API

ARCHITECTURE.md

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+# Reader Suite Architecture (API Backend)
+
+Tai lieu nay mo ta `reader-api` la backend dung chung cho Web (`reader`) va Android (`reader-app`).
+
+## Vai tro trung tam
+
+- `reader-api` la single source of truth cho:
+  - API contract
+  - domain rule
+  - auth mapping web/mobile
+  - data orchestration PostgreSQL + MongoDB
+- Moi thay doi contract phai uu tien backward-compatible cho 2 client.
+
+## Domain ownership
+
+- User domain: profile, settings, bookmarks, reading progress, recommendations.
+- Content domain: genres, novels, chapter list, chapter content.
+- Interaction domain: comments, ratings.
+
+## Data strategy
+
+- PostgreSQL:
+  - user, novel metadata, genres, comments, ratings, bookmarks, progress.
+- MongoDB:
+  - chapter text content lon.
+  - recommendation document payload (neu can rich document).
+
+## Auth and identity
+
+- Web auth: session cookie (NextAuth token va secure variants).
+- Mobile auth: JWT tu `/api/auth/mobile-login`.
+- Backend map ca 2 co che vao cung identity va permission model.
+
+## API compatibility policy
+
+- Khong xoa field dang duoc client dung khi chua co migration plan.
+- Them field moi theo huong optional tru khi co versioning.
+- Error response phai on dinh theo format chung (code, message, details).
+- Versioning uu tien uri prefix hoac contract evolution, tranh breaking ngay.
+
+## Definition of Done (API)
+
+- Endpoint moi co docs request/response + auth requirement.
+- Da verify voi web + mobile happy path va auth edge cases.
+- Healthcheck va monitoring khong bi anh huong.
+- Docker/local dev van chay voi huong dan README.