Update server-audit.md: mark Phase 4 complete (commit b4968d4)

Phase 4: Device communication resilience (batch scanning, retry, configurable timeouts, non-blocking events, reset check)

Phase 3: API hardening (validation, JSON wrappers, path traversal, rate limiting)

Phase 2: Data integrity & error handling (ErrorHandler, transactions, silent SQL, HTTP statuses, dead stubs)

Phase 1: Security Foundation (auth, SQLi, secrets, hashing, tokens, cookies)