feat(admin): grant hints to a user's wallet from the console

Add an "Add hints" form on the admin user card that additively tops up a
player's hint wallet (1-100 per grant). The grant is raise-only by
construction (an additive UPDATE never lowers the balance) and stays correct
under a concurrent in-game spend; a per-grant cap bounds a fat-finger, since
the console can never reduce a wallet.

The in-game hint policy is unchanged and already correct: a game offers the
per-seat allowance plus the wallet, spending the allowance first and the
wallet only after (covered by TestHintPolicy).

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

@@ -21,6 +21,10 @@
 <button type="submit">Clear high-rate flag</button>
 </form>
 {{end}}
+<form class="form" method="post" action="/_gm/users/{{.ID}}/grant-hints">
+<label>Add hints <input type="number" name="amount" min="1" max="{{.HintGrantMax}}" value="1"></label>
+<button type="submit">Grant</button>
+</form>
 </section>
 <section class="panel"><h2>Statistics</h2>
 {{if .HasStats}}