Rate-limit sign-in by visitor behind Cloudflare and stop forwarding h2c

Cloudflare proxies the Progmasoft hosts, so Nginx saw a Cloudflare edge as
its peer and appended that edge to X-Forwarded-For. The API takes the last
hop as the client, so the account-auth limit (12 attempts in 3 minutes) was
counted per edge: 38 failed sign-ins from one address in a row were all
answered with 401, none with 429, and unrelated visitors behind one edge
shared a bucket.

- ops/nginx/cloudflare-real-ip.conf trusts CF-Connecting-IP only from the
  published Cloudflare ranges; a direct peer keeps its own address, so the
  header cannot be forged around Cloudflare. With it installed the 13th
  failed sign-in gets 429, a forged X-Forwarded-For changes nothing, and
  another address is not affected.
- Every proxied location forwarded the client's Upgrade header with
  "Connection: upgrade" on each request. Only a WebSocket upgrade is now
  forwarded; other requests carry no Upgrade and an empty Connection.
- c8 reached glob 13 (GHSA-3g3m-2fjr-6pw4) and minimatch 10
  (GHSA-g3hv-75wg-m249) through test-exclude; both are overridden to the
  fixed majors. pnpm audit reports nothing, coverage, lint and check pass.
- The CodeQL action pins carry their exact version comment (v4.38.2).
- ops/README.md describes the address handling and no longer names a
  script that left the repository with AI Support.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

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