fix(hub): let this origin frame its own download URL

Three headers govern whether a page may be framed, and all three had to be
wrong for the streamed download to work — so fixing them one at a time cost an
afternoon of redeploys and retests. They were visible together in a single
`curl -I` against the deployed hub, which is where this should have started.

The streamed-download path navigates a hidden iframe to `/_mbdl/<id>` so the
service worker is asked for the response it is holding. On Firefox and Safari
that is the only way to write a large file to disk: neither has the File System
Access API, and OPFS is capped at 10% of the volume's size (measured on Firefox
154: 389,233,459 bytes of a 3,892,334,592-byte volume, refused to the byte),
which a film exceeds.

  - `frame-src` was reCAPTCHA's two origins with no `'self'`, so the frame
    could not be loaded at all. Added when the captcha needed a frame; nobody
    connected the two.
  - `frame-ancestors 'none'` forbids all framing, this origin included.
  - `X-Frame-Options: DENY` says the same in an older dialect. The spec says a
    browser must ignore it when frame-ancestors is present — relying on that
    while shipping a header that contradicts our own policy is asking to be
    surprised, and we were: the CSP was fixed and the download stayed broken.

`'self'` and `SAMEORIGIN` refuse every foreign origin exactly as `'none'` and
`DENY` do. The clickjacking property is untouched; what they additionally allow
is this origin framing itself, which is the only thing the download needed.

Pinned three ways: `frame-src` must carry `'self'`, `frame-ancestors` must be
`'none'` or `'self'` and never name an origin, and the two framing headers must
agree — the defect was the disagreement, and either one read as correct alone.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HCGdheDLxGReuKHga3BtST

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