Fix F-08, F-12, F-13: Implement account lockout, username validation, and password minimum length enforcement

- Added failed login tracking for account lockout enforcement in `db` and `ui` layers; introduced `failed_logins` table to store attempts, window start, and attempt count.
- Updated login checks in `grpcserver/auth.go` and `ui/handlers_auth.go` to reject requests if the account is locked.
- Added immediate failure counter reset on successful login.
- Implemented username length and character set validation (F-12) and minimum password length enforcement (F-13) in shared `validate` package.
- Updated account creation and edit flows in `ui` and `grpcserver` layers to apply validation before hashing/processing.
- Added comprehensive unit tests for lockout, validation, and related edge cases.
- Updated `AUDIT.md` to mark F-08, F-12, and F-13 as fixed.
- Updated `openapi.yaml` to reflect new validation and lockout behaviors.

Security: Prevents brute-force attacks via lockout mechanism and strengthens defenses against weak and invalid input.

CLAUDE.md

@@ -72,3 +72,4 @@ This is a security-critical project. The following rules are non-negotiable:
 - `ARCHITECTURE.md` — **Required before any implementation.** Covers token lifecycle, session management, multi-app trust boundaries, and database schema. Do not generate code until this document exists.
 - `PROJECT_PLAN.md` — Discrete implementation steps (to be written)
 - `PROGRESS.md` — Development progress tracking (to be written)
+- `openapi.yaml` - Must be kept in sync with any API changes.