Featured rotation format: duration-priced link rotation with disclosed dilution

- New 'featured' type: headline + link + 1/2/7 day run, flat up-front price (40cr/day)
- Composer discloses dilution (N links share the rotation, yours makes N+1) before buying
- Overview featured strip shows the live rotation; expires automatically

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

db.js

@@ -114,6 +114,7 @@ async function bootstrap() {
   await alterSafe('ALTER TABLE campaigns ADD COLUMN height INT NULL');  // banner size (IAB) → also NAS height
   await alterSafe('ALTER TABLE campaigns ADD COLUMN nas_ad_id INT NULL');          // syndicated NAS sponsorads.ID
   await alterSafe('ALTER TABLE campaigns ADD COLUMN nas_served INT NOT NULL DEFAULT 0'); // NAS impressions already reconciled into spend
+  await alterSafe('ALTER TABLE campaigns ADD COLUMN expires BIGINT NULL'); // featured rotation end time
   await q(`CREATE TABLE IF NOT EXISTS sponsor_messages (
     id INT AUTO_INCREMENT PRIMARY KEY,
     from_member INT NOT NULL,