When behind Cloudflare CDN, RemoteAddr shows CF's IP, breaking rate
limiting and logging. Trust CF-Connecting-IP (set by CF, cannot be
spoofed by clients) and fall back to RemoteAddr for direct connections.
- Add RateLimitMiddleware(10/min) to POST /login (previously unprotected)
- Use RemoteAddr instead of X-Real-IP/X-Forwarded-For in getRemoteIp() and rate limiter
- Prevents brute-force login and rate-limit bypass via spoofed headers
- Add per-IP rate limiter middleware (5 req/min) on /register endpoint
- Validate username (3-64 chars) and password (8-128 chars) with trim
- Use sentinel error ErrUsernameAlreadyExists instead of string matching
- Prevent TurnstileSecretKey exposure via admin settings API (json:"-")
- Skip json:"-" fields in UpdateAllSetting to avoid overwriting secrets
- Add SetTurnstileSecretKey setter for programmatic configuration
- Reuse package-level http.Client in Turnstile verification for connection pooling
- Add io.LimitReader to cap Turnstile response body size
- Log all Turnstile verification error paths for debugging
- Add invalidUsername/invalidPassword i18n keys to all 13 locales
