test(users): UsersService unit tests and controller integration tests; structured logging and error handling

[bobaivigitalpoint-ui]

• Add unit tests for UserAPI #1347: src/users/users.service.spec.ts: unit tests for create (hash, secrets stripped, referral logic), lookups, block/verify/remove, referrals and login history; docs/api/users-tests.md
• Add integration tests for UserAPI #1348: test/e2e/users-controller.integration.spec.ts: UsersController over HTTP with the main.ts ValidationPipe settings, mocked services and role-aware guards
• Improve error handling in UserAPI #1349: GET /users/:id now returns 404 for an unknown or deactivated user instead of an empty 200; docs/api/users-error-handling.md
• Add structured logging to UserAPI #1350: UsersService moves to AppLogger with stable event names and metadata; removes a raw email from the reactivation-token log; docs/api/users-structured-logging.md

Scope kept minimal: small module-local additions plus a short doc under docs/api/ per issue. Each issue's tests are not added unless the issue is itself about tests; nothing was run. The new tests were not run; expectations were written from the current service/controller code. Other User API PRs in this batch also edit users.service.ts and users.controller.ts, so expect trivial merge conflicts.

Nothing was installed, built or run (no tsc, lint or jest); please verify in CI/review.

Closes #1347
Closes #1348
Closes #1349
Closes #1350

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[nanaf6203-bit]

LGTM