Ad engine v1: banner/text/login campaigns against on-chain credits

Spec 8b types 1-3. Spend accrues per campaign in batches; burns queue for
the engine signer (admin runs consume() on-chain, /api/admin/burns). Rates
are volume config (adrates.json), rehearsal placeholders until Marty sets
the real card. Public slots serve on the ledger page; campaign manager in
the members area.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

public/assets/ledger.js

@@ -15,6 +15,9 @@
     IAP.$('statLine').textContent = stats.onchainMembers + ' on-chain member(s)';
   } catch (e) {}
 
+  IAP.adSlot('banner', 'adSlotBanner');
+  IAP.adSlot('text', 'adSlotText');
+
   const es = new EventSource('/api/feed/live');
   es.onopen = () => { const b = IAP.$('liveBadge'); b.textContent = '● live'; };
   es.onerror = () => { const b = IAP.$('liveBadge'); b.textContent = 'reconnecting…'; };