feat: add webhook_subscription table and CRUD endpoints

- Migration: CREATE TABLE webhook_subscription (id, did, method, url) with index on did
- db.ts: addWebhookSubscription and deleteWebhooksByDid helpers
- server.ts: POST /:did/webhooks (201) and DELETE /:did/webhooks (204)

src/migrations/init.ts

@@ -18,4 +18,18 @@ db.exec(`
   );
 `);
 
+db.exec(`
+  CREATE TABLE IF NOT EXISTS webhook_subscription (
+    id     INTEGER PRIMARY KEY,
+    did    TEXT NOT NULL,
+    method TEXT NOT NULL,
+    url    TEXT NOT NULL
+  );
+`);
+
+db.exec(`
+  CREATE INDEX IF NOT EXISTS idx_webhook_subscription_did
+    ON webhook_subscription(did);
+`);
+
 db.close();