Add rate limiting and monitoring to admin login

[AJFrio]

Summary

• track admin login attempts in KV by IP and user-agent with configurable thresholds, slowdown, and lockout handling
• normalize error handling to return generic messages while logging and monitoring suspicious activity
• record failure metadata in KV and console logs so repeated attacks are visible to administrators

Testing

• npm run lint (fails: existing lint errors in unrelated files)

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https://chatgpt.com/codex/tasks/task_b_68dd7a7ffb9c8329a90cd3df0ad7cde5