fix(backend): raise compress threshold to fix empty zstd /api/health body

[laboef1900]

Summary

• /api/health was returning an empty body with content-encoding: zstd and content-length: 0 to the browser in the deployed EE backend container — Settings → Diagnostics consequently fell back to "Edition: Community (CE)" and "Backend commit: …" because await response.json() parsed an empty body.
• Larger payloads on the same backend (/api/pages/:id at 1614 bytes) compressed fine. The bug is deterministic on the deployed container but does not reproduce against @fastify/compress 8.3.1 in isolation on Node 26 with the same payload shape — almost certainly an interaction with the container's Node 24 zstd encoder or the EE build pipeline. Investigation hit diminishing returns.
• Workaround: register compress with { threshold: 4096 }. Payloads in the bug-prone size range pass through uncompressed; payloads where compression actually saves meaningful bandwidth still get compressed.

Repro before the fix

curl -sH "Accept-Encoding: gzip, br, zstd" -i http://localhost:3052/api/health | head -30
# → 200 OK
# → content-encoding: zstd
# → content-length: 0
# → (empty body)

After the fix the same request returns the full 1042-byte JSON with no content-encoding header.

Test plan

• Two new regression tests in app.test.ts:
  • sub-4KB synthetic payload → no content-encoding, body intact

  • 4KB synthetic payload → some content-encoding set, body non-empty (so the threshold doesn't accidentally disable compression entirely)

• All existing app tests pass (11/11)
• Typecheck clean
• Manual verification after EE backend rebuild — Diagnostics should show "Edition: Enterprise (EE)" + real backend commit hash

Follow-up

The EE backend image needs to be rebuilt for this fix to take effect in the running stack. Tracking the underlying @fastify/compress / Node 24 zstd interaction in a separate issue.

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