The real server lives in a separate repo; this repo carries reference
stubs for each endpoint (see camera_endpoint.py precedent). Adds the
Pydantic extension, persistence helper, migration 005, and tests that
the real server can copy when adding diagnostic-field support.
Matches the firmware v1.1.0 heartbeat payload shape. Old-shape
payloads (firmware v1.0.0) continue to parse cleanly with the new
fields defaulting to None.
