feat: AI-game refinements (GCG, your_turn, admin, metrics)

Follow-ups on the honest-AI game, same PR:
- GCG export labels the robot seat "AI" instead of its pool name (ExportGCG
  overrides via accounts.IsRobot); the in-app 🤖 is unchanged.
- vs_ai games emit no your_turn (the robot replies instantly, so it would be
  redundant); opponent_moved still advances the UI.
- Admin console shows the AI flag: a 🤖 column in /games and an "AI game" line
  on the game card (GameRow/GameDetailView gain VsAI).
- games_started_total / games_abandoned_total gain a vs_ai attribute; the
  Grafana Game-domain dashboard splits started/abandoned into human and AI
  panels.

Tests: metrics unit (vs_ai split); integration (no your_turn, GCG "AI").

PRERELEASE.md

@@ -503,3 +503,11 @@ Then Stage 18.
   - **Schema/wire change → a contour DB wipe** after merge (`DROP SCHEMA backend CASCADE` + restart, the
     R1/R3 pattern). Bake-back: `docs/ARCHITECTURE.md`, `docs/FUNCTIONAL.md` (+`_ru`), `docs/UI_DESIGN.md`,
     `backend/README.md`, Go Doc comments.
+  - **Post-review refinements (owner, same PR):** (1) the **GCG export labels the robot seat "AI"** rather
+    than its human-like pool name (`ExportGCG` overrides the name via `accounts.IsRobot`; the in-app 🤖 is
+    unchanged); (2) honest-AI games **emit no `your_turn`** — the robot replies instantly, so the signal
+    would arrive with the move and be pointless; `opponent_moved` still advances the UI; (3) the **admin
+    console surfaces the AI flag** — a **🤖 column** in `/games` and an "AI game" line on the game card
+    (`GameRow`/`GameDetailView` gain `VsAI`); (4) `games_started_total` / `games_abandoned_total` gain a
+    **`vs_ai`** attribute and the Grafana *Game domain* dashboard splits started/abandoned into **human**
+    and **AI** panels.