doc: code architecture doc + contributing guide for backend (#2953)

* architecture doc

* agents and contributor md guides

* fix: userStore

src/backend/package.json

@@ -73,6 +73,7 @@
     "validator": "^13.15.35"
   },
   "devDependencies": {
+    "@types/bcrypt": "^6.0.0",
     "@types/busboy": "^1.5.4",
     "@types/node": "^24.0.0",
     "chai": "^4.3.7",

src/backend/stores/user/UserStore.ts

@@ -43,6 +43,7 @@ export interface UserRow {
     requires_email_confirmation?: boolean;
     /** Metadata JSON blob; decoded on read when the DB returns it as a string. */
     metadata?: Record<string, unknown>;
+    password?: string;
     [k: string]: unknown;
 }