feat(server): public GET /health endpoint for uptime monitors

Returns 200 with {status: ok, version, db: ok} when SQLite is reachable,
503 when the DB probe fails. Unauthenticated so external monitors can
poll without credentials.

server/test/server_web/controllers/health_controller_test.exs

@@ -0,0 +1,11 @@
+defmodule ServerWeb.HealthControllerTest do
+  use ServerWeb.ConnCase, async: true
+
+  test "GET /health returns 200 with status=ok", %{conn: conn} do
+    conn = get(conn, ~p"/health")
+    body = json_response(conn, 200)
+    assert body["status"] == "ok"
+    assert body["db"] == "ok"
+    assert is_binary(body["version"])
+  end
+end