fix: Align account lockout threshold with IP rate limiting

Fixed inconsistency between IP-based rate limiting and per-account lockout.
Previously, users would hit the IP rate limit at 5 attempts (15 min lockout)
but the account wouldn't be marked as locked until 10 attempts (30 min).
This caused confusion in the admin UI where locked accounts wouldn't show
the unlock button until 10 attempts were reached.

Changes:
- Reduced account lockout threshold from 10 to 5 failed attempts
- Reduced account lockout duration from 30 to 15 minutes
- Updated error message to reflect 15 minute lockout period
- Added detailed logging when account gets locked
- Updated README documentation to reflect correct limits

Both protection layers now work in harmony:
- IP-based rate limiting: 5 attempts = 15 min lockout
- Per-account lockout: 5 attempts = 15 min lock

This ensures the admin UI accurately shows account lock status and provides
the unlock option as soon as users hit the lockout threshold.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

server/api/auth/login.post.ts

@@ -57,7 +57,7 @@ export default defineEventHandler(async (event) => {
     console.log(`[LOGIN BLOCKED] Account locked - User: ${username.toLowerCase()}, IP: ${clientIp}`)
     throw createError({
       statusCode: 403,
-      message: 'Account temporarily locked due to too many failed login attempts. Please try again in 30 minutes or contact an administrator.'
+      message: 'Account temporarily locked due to too many failed login attempts. Please try again in 15 minutes or contact an administrator.'
     })
   }