Stage 17 round 6 (#18, PR D): admin Messages moderation section

A new /_gm/messages console page lists posted chat messages (nudges
excluded) newest-first — time, source (guest/robot/oldest identity kind),
sender (linked to the user card), IP, body, game (linked to the game card)
— searchable by sender name / external-id glob masks and pinnable to one
game (?game=) or sender (?user=), linked from the game and user cards.

The list query lives in social (raw SQL, kind='message', source via a SQL
CASE), reusing the now-exported account.LikePattern. Server-rendered
adminconsole MessagesView + messages.gohtml, 50/page via the shared pager.

Tests: adminconsole render case; backend integration AdminListMessages
(real Postgres) — nudge exclusion, game/sender pins, glob masks, source.
Docs: ARCHITECTURE section 8 chat moderation, PLAN round-6.

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

@@ -1,7 +1,7 @@
 {{define "content" -}}
 {{with .Data}}
 <h1>{{.DisplayName}}</h1>
-<nav class="subnav"><a href="/_gm/users">&laquo; users</a></nav>
+<nav class="subnav"><a href="/_gm/users">&laquo; users</a> · <a href="/_gm/messages?user={{.ID}}">messages</a></nav>
 <div class="cards">
 <section class="panel"><h2>Account</h2>
 <ul class="kv">