handbook alignment: profile.updated mirror, health contract, Synapse gateway ...

Handbook gaps closed (10-platform auth/health/notifications):

- webhooks: handle user.profile_updated — mirror the gnexus-auth profile
  into navi_users (name, contact fields, avatar_url — new column, boot
  migration); role/permissions keep their dedicated events
- auth: avatar_url now flows through the login upsert and the API-token
  resolution path
- /health: status is now the aggregate of the sub-checks (degraded embed
  or hive no longer reports plain ok); embed probe cached for 10 s; body
  grew the machine-readable checks{} map, legacy embed/hive payloads kept
- Synapse: s2s delivery gateway — POST /webhooks/synapse (both spellings),
  per-user delivery secrets (synapse_targets, Fernet-encrypted, shown
  once), verification via gnexus-synapse v0.1.2 client lib, idempotent
  event ids; deliveries are verified and logged for now — navi generates
  no outbound events by decision; web-push stays as the local browser
  channel
- webclient settings: Synapse deliveries panel (add/revoke targets)

security: critical batch 1 — RCE/XSS/CORS/webhook hardening ...

Backend:
- auth/deps: fix refresh-lock clock race (cleanup now monotonic like the cache)
- core/registry: add missing structlog logger (NameError on fallback path)
- config: GNAUTH_WEBHOOK_SECRET, NAVI_ALLOWED_ORIGINS (+list property)
- webhooks: HMAC-SHA256 signature verification (503 unconfigured in auth mode,
  unsigned+warning in no-auth mode, 403 bad/stale signature, 400 bad JSON)
- main: CORS from NAVI_ALLOWED_ORIGINS in auth mode (fail-fast on empty),
  eval router behind require_admin, /debug only registered in no-auth mode
- auth routes: mobile-done sid validation (32 hex) + CSP header + safe JS
  escaping (reflected XSS); Secure cookie flag via helper (https base URL)
- websocket: anonymous WS rejected in auth mode (closes legacy-session RCE);
  socket registered only after access checks; stop_session auth gate
- messages: REST agent start now takes session lock + busy flag (409 on
  active run), contextvars reset in finally

Webclient:
- useMarkdown: DOMPurify.sanitize on all rendered markdown (stored XSS),
  image-URL scheme whitelist, delegated error listener (no inline onerror)
- html.html artifact viewer: sandbox without allow-same-origin (opaque origin)
- tests: DOMPurify runs under jsdom (happy-dom Node.prototype.nodeName getter
  breaks DOMPurify); useWebSocket tests get localStorage stub + ui-kit alias

Tests: pytest 1049 passed, 1 skipped; vitest 61 passed