Release 0.2.0: changelog, version reporting, push-to-deploy

- CHANGELOG.md following Keep a Changelog, covering 0.1.0 and 0.2.0
- server/version.js reads the version from package.json; /healthz returns it
  and the footer renders it, so a deployment always states what it is serving
- bump package.json 0.1.0 -> 0.2.0

server/version.js

@@ -0,0 +1,7 @@
+// Single source of truth for the running version: package.json, read once at start.
+import { readFileSync } from 'node:fs';
+import { fileURLToPath } from 'node:url';
+import { dirname, join } from 'node:path';
+
+const here = dirname(fileURLToPath(import.meta.url));
+export const VERSION = JSON.parse(readFileSync(join(here, '..', 'package.json'), 'utf8')).version;