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>

goodnews/api.py

@@ -588,13 +588,21 @@ class SourceReviewBody(BaseModel):
 _FEEDBACK_CATEGORIES = {"idea", "concern", "bug", "praise", "other"}
 
 # The only event kinds we record. All aggregate, non-personal.
+# Per-game funnel events (article_id is reused as 0 — no article dimension). Per-game
+# kinds (not a generic "game_started") so the admin kind-count breakdown shows which
+# game drives play and, crucially, shares — the growth loop we're instrumenting.
+_GAME_NAMES = ("word", "wordsearch", "bloom", "match")
+# arrival = landed on the game via a shared link (utm_source=game_share) — the share
+# loop's acquisition signal; started/completed/shared are the engagement funnel.
+_GAME_EVENT_KINDS = {f"{g}_{e}" for g in _GAME_NAMES for e in ("started", "completed", "shared", "arrival")}
+
 _EVENT_KINDS = {
     "visit", "open", "summary_viewed", "full_story", "source_click",
     "share_ub", "copy_source", "native_share",
     "not_today", "less_like_this", "hide_topic",
     "replace_used", "replace_none", "paywall_replace", "paywalled_source_open",
     "client_error",  # boot-failure seatbelt beacon (blank-screen risk signal)
-}
+} | _GAME_EVENT_KINDS
 
 
 def _fts_query(q: str) -> str: