Admin step A: privacy-respecting first-party event logging

- events table (kind, article_id, visitor_hash, day) with a UNIQUE key that dedups
  to one row per visitor-day — caps volume and makes counts mean distinct
  visitor-days. NO ip/ua/referrer/url. Groupings derived from article_id at query
  time, never stored.
- POST /api/events (public): whitelisted kinds (visit/open/share_ub/copy_source/
  native_share/source_click); visitor token hashed server-side (never raw).
- Frontend analytics.js: random localStorage visitor token; track() via sendBeacon;
  visit once/day; open on article click; share_ub/copy_source/native_share from the
  share menu; /a landing pages fire source_click. 127 tests pass.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/routes/+page.svelte

@@ -10,6 +10,7 @@
   import BoundariesPanel from '$lib/components/BoundariesPanel.svelte';
   import SignIn from '$lib/components/SignIn.svelte';
   import { auth, savedIds, refresh as refreshAuth } from '$lib/auth.svelte.js';
+  import { trackVisit } from '$lib/analytics.js';
 
   let moods = $state([]);
   let topics = $state([]);
@@ -302,6 +303,7 @@
     dismissed = new Set(P.loadJSON(DISMISSED_KEY, []));
     history = P.loadJSON(HISTORY_KEY, []);
     refreshAuth(); // resolve any existing session (non-blocking)
+    trackVisit(); // anonymous, once/day
     try {
       moods = await getJSON('/api/moods');
       topics = (await getJSON('/api/categories')).topics;