feat(backend): per-tier, per-kind active-game limits with a guest funnel

Cap a player's simultaneous unfinished games per kind (vs_ai, random,
friends) with independent guest and durable-account tiers, held in a new
single-row backend.config table (-1 = unlimited) behind an in-memory cache
and editable live in the admin console (/_gm/limits). Each game is tagged
with games.game_kind on creation.

This replaces the earlier flat MaxActiveQuickGames=10 combined cap: the
per-tier/kind config is the single mechanism, enforced at the same handler
gate (ensureUnderGameLimit by kind on lobby/enqueue) plus the durable
friends cap in CreateInvitation. game.Service.AtGameLimit only resolves the
tier and counts; the limit policy stays at the request edge.

Guests are now refused friend requests, friend-code redemption,
befriend-in-game and invitation creation outright (403 guest_forbidden) --
previously only the UI hid these.

Admin: a kind column in both game lists and the config editor.

Defaults: guest 1 vs_ai / 1 random / 0 friends; durable 10 / 10 / 10.

backend/internal/adminconsole/templates/pages/user_detail.gohtml

@@ -211,11 +211,11 @@
 {{end}}
 <section class="panel"><h2>Games</h2>
 <table class="list">
-<thead><tr><th>Game</th><th>Variant</th><th>Status</th><th class="num">Players</th><th>Updated</th></tr></thead>
+<thead><tr><th>Game</th><th>Variant</th><th>Kind</th><th>Status</th><th class="num">Players</th><th>Updated</th></tr></thead>
 <tbody>
 {{range .Games}}
-<tr><td><a href="/_gm/games/{{.ID}}">{{.ID}}</a></td><td>{{.Variant}}</td><td>{{.Status}}</td><td class="num">{{.Players}}</td><td>{{.UpdatedAt}}</td></tr>
-{{else}}<tr><td colspan="5"><span class="note">no games</span></td></tr>{{end}}
+<tr><td><a href="/_gm/games/{{.ID}}">{{.ID}}</a></td><td>{{.Variant}}</td><td>{{.Kind}}</td><td>{{.Status}}</td><td class="num">{{.Players}}</td><td>{{.UpdatedAt}}</td></tr>
+{{else}}<tr><td colspan="6"><span class="note">no games</span></td></tr>{{end}}
 </tbody>
 </table>
 </section>