feat: withdrawals API docs, enhanced tests, notifications POST /:id/read

[Lekan101]

Summary

Implements 4 backend issues against the test-implement-drips branch:

#945 — Withdrawals API docs

• Added full OpenAPI documentation for all withdrawals and balances endpoints in withdrawals.routes.ts
• Added Withdrawals tag to the shared OpenAPI document
• Covers: GET /withdrawals/me, POST /withdrawals/prepare, POST /withdrawals/submit, GET /balances/me

#944 — Withdrawals module tests

• Added edge-case tests: empty withdrawal history, custom pagination params, null serialization, zero balance, 401 on unauthenticated access, empty body validation, insufficient balance on prepare

#958 — Notifications module skeleton

• Verified the notifications module skeleton is complete (service, controller, routes, schema, types, tests)

#960 — POST /notifications/:id/read

• Added POST /:id/read route as an alias for the existing PATCH /:id/read endpoint
• Added OpenAPI docs for the POST variant
• Added comprehensive tests for the POST route (success, 404, 401, cross-user)

Verification

• npm run lint passes clean (0 errors, only pre-existing warnings)
• npm run typecheck — pre-existing errors only (missing @stellar/stellar-sdk types, socket.io types)

Closes

Closes #944, Closes #945, Closes #958, Closes #960