sentry: tunnel client events through the app to dodge Cloudflare's 503

GlitchTip is behind Cloudflare, which answers the browser's cross-site ingest
POSTs with 503 (no CORS headers -> 'Failed to fetch'); server-side requests
pass. The browser SDK now posts envelopes same-origin to /sentry-tunnel, and
the server forwards them to GlitchTip:
- imports/startup/server/sentryTunnel.js: WebApp handler that relays the raw
  envelope to <dsn>/api/<project>/envelope/?sentry_key=<key> over IPv4
  (family:4 avoids Happy-Eyeballs picking Cloudflare's unroutable IPv6 on
  IPv4-only hosts). GlitchTip authenticates by the sentry_key query, so the
  key is derived from the DSN and passed explicitly.
- client ravenLogger: tunnel: '/sentry-tunnel'.
Verified: POST /sentry-tunnel -> 200 (envelope reaches GlitchTip).

imports/startup/server/index.js

@@ -1,4 +1,5 @@
 import './ravenLogger';
+import './sentryTunnel';
 import './catchExceptions';
 import './i18n';
 import './accounts';