Test 607 — Hub authoritative upload limits

Issue: https://github.com/sleep2agi/agent-network/issues/496
Base commit: 5b2688fdc41e8c5bfa57820645abcf1f09ffe3df
Source commit: a90c0cf64231702f2c06dd309471653314d579a6

Scope
- Adds an anonymous, additive `limits` object to `GET /health`.
- Values are sourced from the same server constants that enforce upload and request limits.
- Existing health fields and authentication behavior are unchanged.

Docker evidence
- Image: anet-test607-hub:dev
- Image ID: sha256:b34d34c6b768ee280dedf5e719e7a8e445fe7d1997b461c0fc8aa8e53a39d895
- Embedded TEST607_SOURCE_COMMIT: a90c0cf64231702f2c06dd309471653314d579a6
- Artifact SHA256: 4c792b5a4d0fe6b20cb8241f064130e914df38362e5cbc1c799de49134b3c656

Results
- Real Hub build: PASS (257 modules, 1.52 MB output)
- Real HTTP health tests: PASS (2 tests, 9 assertions)
- Anonymous health response retains watchdog-compatible shape: PASS
- `max_upload_bytes` equals the enforced Hub upload constant: PASS
- `max_request_content_length` equals the enforced request constant: PASS
- Request limit is not lower than upload limit: PASS

Witnessed-red mutation
- Mutation: replace the exact health response binding
  `max_upload_bytes: MAX_UPLOAD_BYTES` with `max_upload_bytes: 1`.
- Result: FAIL (rc=1) on the public health limit assertion.
- Restored source: PASS (2 tests, 9 assertions).

Final result: PASS
