Rewrite backend in Go with JSON API and JS frontend

Reimplements the Flask app as a Go HTTP API (chi + modernc sqlite) with a
minimal vanilla-JS frontend in web/. Endpoints mirror the original Flask
routes but return JSON instead of HTML.

- auth: login/logout/register/sessioninfo/headerbar with crypto/rand tokens
- entry: paginated feed (single JOIN), create with image scaling
- user: profile, userinfo; delete still a stub
- requireAuth middleware passes uid via context
- notes/api.md documents the API and schema

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.gitignore

@@ -1,5 +1,6 @@
 venv/
 kver.db
+/kver
 *.env
 __pycache__/
 static/media/