feat(server): heartbeat-diagnostics stub + migration for real server import

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.

server/migrations/005_heartbeat_diagnostics.sql

@@ -0,0 +1,14 @@
+-- migrations/005_heartbeat_diagnostics.sql
+-- Add v1.1.0 diagnostic columns to the existing heartbeats table.
+-- Adjust the table name ("heartbeats") to match the real server's schema.
+-- Apply: sqlite3 <db_file> < migrations/005_heartbeat_diagnostics.sql
+--
+-- sqlite's ALTER TABLE ADD COLUMN only takes one column per statement, so
+-- each field is added separately. All columns are nullable, so firmware
+-- v1.0.0 payloads (which omit these fields) remain accepted unchanged.
+
+ALTER TABLE heartbeats ADD COLUMN reset_reason         INTEGER;
+ALTER TABLE heartbeats ADD COLUMN heap_free            INTEGER;
+ALTER TABLE heartbeats ADD COLUMN heap_min_free        INTEGER;
+ALTER TABLE heartbeats ADD COLUMN last_disconnect_code INTEGER;
+ALTER TABLE heartbeats ADD COLUMN recent_events        TEXT;    -- JSON-serialized list of {t,d0,d1,ts,up}