feat(webhooks): add per-verb subscriptions and bulk-create debounce

Subscriptions now declare a `verb` (create | delete | bulk-create).
POST /:did/webhooks defaults to inserting both create and delete rows
when no verb is given, preserving existing all-events behavior. Update
events fold into the create verb. The new bulk-create verb debounces
creates per DID over 400 ms and delivers a `records` array.

Migration adds the verb column with default 'create' and clones every
existing row for the delete verb so legacy subscriptions keep firing
on both events.

src/migrations/init.ts

@@ -61,4 +61,23 @@ try {
   // Column already exists — no-op
 }
 
+try {
+  db.exec(
+    `ALTER TABLE webhook_subscription ADD COLUMN verb TEXT NOT NULL DEFAULT 'create';`,
+  );
+  db.exec(`
+    INSERT INTO webhook_subscription (did, method, url, token, verb)
+    SELECT did, method, url, token, 'delete'
+    FROM webhook_subscription
+    WHERE verb = 'create';
+  `);
+} catch {
+  // Column already exists — backfill already happened on a previous run.
+}
+
+db.exec(`
+  CREATE INDEX IF NOT EXISTS idx_webhook_subscription_did_verb
+    ON webhook_subscription(did, verb);
+`);
+
 db.close();