Game share-loop: instrument funnel, deep-link shares, /play metadata

Sharpen the existing daily-game share loop into something measurable (per Codex's
"instrument what you have, then feed people into it" plan), ahead of a Show HN launch.

Analytics:
- Per-game funnel events <game>_{arrival,started,completed,shared} (article_id=0).
  arrival = landed via a shared link (utm_source=game_share); started = first move
  (guess/find/flip); completed = solved/cleared/Full Bloom; shared = on share success.
- trackVisit() moved into the global layout so direct /play landings count; the
  server-rendered /a/ share page now creates a visitor token + sends a daily visit
  beacon (first-time /a/-only visitors were previously dropped).
- Admin "Games funnel" panel: arrivals / engaged / completed / shared, per game.

Sharing:
- Memory Match gains a Share button (it was the only game without one).
- All shares deep-link to the exact game+variant with a full https:// URL +
  utm_source=game_share (gameShareUrl helper), instead of a bare /play.
- "shared" is counted only after navigator.share()/clipboard.writeText() succeeds.

/play social metadata:
- /play served homepage canonical/OG (static SPA, ssr=false). postbuild script
  patches build/play.html's head to /play canonical/title/description/OG; fails the
  build if the homepage tags drift. Caddy try_files now serves {path}.html so /play
  is served from the patched file (snapshot in deploy/caddy/).

Tests: backend 352, frontend 27.

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

tests/test_share.py

@@ -51,6 +51,16 @@ def test_share_page_duplicate_redirects_to_canonical(client):
     assert r.status_code == 301 and r.headers["location"] == "/a/1"
 
 
+def test_share_page_creates_visitor_token_for_beacons(client):
+    # /a/ is server-rendered outside the SPA, so its inline analytics must CREATE the
+    # visitor token when absent (mirroring visitorId) — else a first-time /a/ landing
+    # beacons an empty token and is dropped from visitor stats.
+    html = TestClient(client).get("/a/1").text
+    assert "localStorage.setItem('goodnews:visitor'" in html
+    assert "crypto.randomUUID" in html
+    assert "kind:'visit'" in html              # daily visit beacon present
+
+
 def test_share_page_no_image_uses_summary_card(client, tmp_path, monkeypatch):
     # article 1 has an image → large card
     assert 'summary_large_image' in TestClient(client).get("/a/1").text