Implement analytics feature

Add server-side visit analytics collection, a public-page ping endpoint,
and a full-screen AnalyticsView for admins.

Backend:
- Add pagerite/analytics.py: Analytics/Visit model, Store, and persistence
- Wire /_a ping endpoint and GET /_api/analytics into pagerite/app.py

Frontend:
- Add full-screen AnalyticsView with visitor charts and transition map
- Add VisitorCharts and TransitionGraph subcomponents
- Add analytics JS helpers in frontend/src/analytics/
- Send navigation pings from frontend/src/pagerite.js
- Mount AnalyticsView from frontend/src/main.js
- Document the feature in docs/analytics.md and update AGENTS.md

AGENTS.md

@@ -15,8 +15,9 @@ Pagerite is a CMS. See `docs` for the full design and implementation details. Ke
   - `markdown.py` — markdown-it-py renderer.
   - `views.py` — shared page layout and rendering.
   - `seed.py` — demo content, written only on first database creation.
+  - `analytics.py` — visit analytics collection (see `docs/analytics.md`).
 - `frontend/src/` — Vue editor and public-page JS entries.
-  - `main.js` — Vue editor app entry.
+  - `main.js` — Vue editor app entry (also mounts the full-screen AnalyticsView).
   - `pagerite.js` — public page entry.
   - `assets/` — base CSS, Pygments styles, fonts.
 - `scripts/devserver.py` — dev server with auto reload (the user mostly uses this; avoid running the server yourself, ask the user to test).