feat: Add architecture and contract documentation for reader-app, including API standards and cross-repo mapping

CONTRACT.md

@@ -0,0 +1,64 @@
+# API Contract Standard (Reader Suite)
+
+Tai lieu contract chung cho `reader`, `reader-app`, `reader-api`.
+
+## Base and Versioning
+
+- Base path: `/api/*`
+- Current mode: unversioned with backward-compatible evolution.
+- Breaking changes bat buoc qua ke hoach migration va release note.
+
+## Response Convention
+
+- Success:
+  - `200/201`: tra JSON payload domain data.
+  - `204`: khong co body.
+- Error (standardized):
+
+```json
+{
+  "code": "string",
+  "message": "human readable",
+  "details": {}
+}
+```
+
+## HTTP Status Usage
+
+- `400`: input/validation khong hop le.
+- `401`: chua dang nhap hoặc token/session invalid.
+- `403`: da dang nhap nhung khong du quyen.
+- `404`: resource khong ton tai.
+- `409`: xung dot du lieu.
+- `422`: payload format dung JSON nhung khong dat rule nghiep vu.
+- `500`: loi he thong.
+
+## Pagination Convention
+
+- Query params: `page`, `limit`.
+- Response envelope:
+
+```json
+{
+  "items": [],
+  "pagination": {
+    "page": 1,
+    "limit": 20,
+    "total": 0,
+    "totalPages": 0
+  }
+}
+```
+
+## Auth Matrix
+
+- Web: NextAuth session cookie.
+- Mobile: Bearer JWT.
+- Backend phai map ca 2 vao cung user identity + role.
+
+## Compatibility Rules
+
+- Khong xoa hoac doi y nghia field dang duoc client su dung.
+- Field moi phai optional theo default trong giai doan rollout.
+- Doi ten field => tao migration layer hoac add field moi, deprecate sau.
+- Endpoint moi can update README + mapping matrix.