feat(server): add OTA check and firmware download endpoints

Implements /ota/check (version comparison + sig_b64 payload) and
/ota/firmware (binary stream) using the same _impl pattern as
camera_endpoint.py. HMAC auth left commented pending main app wiring.
6/6 tests passing.

Co-Authored-By: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>

server/test_ota_endpoint.py

@@ -0,0 +1,70 @@
+# server/test_ota_endpoint.py
+import base64
+import hashlib
+import json
+from pathlib import Path
+
+import pytest
+
+from server.ota_endpoint import ota_check_impl, ota_firmware_impl
+
+
+def write_firmware(firmware_dir: Path, version: str, data: bytes = b"fake_fw") -> None:
+    sig = bytes(64)  # zero sig (not validated server-side)
+    manifest = {
+        "version": version,
+        "size": len(data),
+        "sha256": hashlib.sha256(data).hexdigest(),
+    }
+    (firmware_dir / "current.bin").write_bytes(data)
+    (firmware_dir / "current.sig").write_bytes(sig)
+    (firmware_dir / "manifest.json").write_text(json.dumps(manifest))
+
+
+@pytest.fixture(autouse=True)
+def patch_firmware_dir(tmp_path, monkeypatch):
+    import server.ota_endpoint as mod
+    monkeypatch.setattr(mod, "FIRMWARE_DIR", tmp_path)
+    yield tmp_path
+
+
+def test_check_no_update_same_version(tmp_path):
+    write_firmware(tmp_path, "1.0.0")
+    result = ota_check_impl(current_version="1.0.0", firmware_dir=tmp_path)
+    assert result["update"] is False
+
+
+def test_check_no_update_newer_local(tmp_path):
+    write_firmware(tmp_path, "1.0.0")
+    result = ota_check_impl(current_version="1.1.0", firmware_dir=tmp_path)
+    assert result["update"] is False
+
+
+def test_check_update_available(tmp_path):
+    write_firmware(tmp_path, "1.1.0", data=b"new firmware")
+    result = ota_check_impl(current_version="1.0.0", firmware_dir=tmp_path)
+    assert result["update"] is True
+    assert result["version"] == "1.1.0"
+    assert result["size"] == len(b"new firmware")
+    assert "sha256" in result
+    assert "sig_b64" in result
+    sig_bytes = base64.b64decode(result["sig_b64"])
+    assert len(sig_bytes) == 64
+
+
+def test_check_no_manifest(tmp_path):
+    result = ota_check_impl(current_version="1.0.0", firmware_dir=tmp_path)
+    assert result["update"] is False
+
+
+def test_firmware_endpoint_returns_binary(tmp_path):
+    fw_data = b"firmware binary content"
+    write_firmware(tmp_path, "1.1.0", data=fw_data)
+    content = ota_firmware_impl(firmware_dir=tmp_path)
+    assert content == fw_data
+
+
+def test_firmware_endpoint_missing_raises(tmp_path):
+    import server.ota_endpoint as mod
+    with pytest.raises(mod.FirmwareNotFoundError):
+        ota_firmware_impl(firmware_dir=tmp_path)