POST /settings/joint-zero sets one joint's offset so the model reads a known mechanical reference #445

Part of #439 (calibrate the model to the arm). Today jointOffsetsDeg is {} and only PUT /settings can set it, by hand.

Done:

• [ ] POST /settings/joint-zero {"joint": "shoulder_pan", "referenceDeg": 0} on src/server.ts reads that joint's Present_Position. It sets jointOffsetsDeg[joint] so the model angle equals referenceDeg, saves the settings the way table-here does, and returns the new settings plus the offset it applied.
• [ ] The input is validated: a known joint name and a finite referenceDeg within the joint's limits. An offset past ±15° is refused with 409 and a sentence, because it means the arm is not at the reference.
• [ ] The relay treats it as owner-only (ADMIN_ONLY in src/relay.ts).
• [ ] Tests with the fake bus: the offset is applied and saved, then /status and the model agree with the reference within 0.5°. A bad joint is a 400, and an offset that is too large is a 409.
• [ ] bun --cwd Software/so101 test passes.

Risk: offsets feed the safe-zone geometry (server.ts toQ). A test must prove that a zone block still fires at the same physical pose after zeroing.