fix: align docs and code with actual DB schema and format spec

- Rewrite db.md as canonical schema: add categories, sessions tables; add alias to accounts, is_category_confirmed/comment to transactions, FK references to categories(id); mark budgets as post-MVP

- Fix account masking to use fixed 6 asterisks (code + docs)

- Remove budgets from MVP requirements in agent_backend.md

- Add explicit 'not in MVP' note to analytics.md budgets section

- Fix test_Statement.json: convert amounts to kopecks (integers), remove fingerprint fields (computed by backend)

Made-with: Cursor

backend/src/utils.ts

@@ -2,7 +2,7 @@ import { Request, Response, NextFunction } from 'express';
 
 export function maskAccountNumber(num: string): string {
   if (num.length <= 10) return num;
-  return num.slice(0, 6) + '*'.repeat(num.length - 10) + num.slice(-4);
+  return num.slice(0, 6) + '******' + num.slice(-4);
 }
 
 export function escapeLike(input: string): string {