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/data/db.ts

@@ -67,6 +67,31 @@ export const saveCursor = (cursor: number) => {
   );
 };
 
+type WebhookSubscriptionRow = {
+  id: number;
+  did: string;
+  method: string;
+  url: string;
+};
+
+export const addWebhookSubscription = (
+  { did, method, url }: Omit<WebhookSubscriptionRow, "id">,
+): WebhookSubscriptionRow => {
+  db.exec(
+    "INSERT INTO webhook_subscription (did, method, url) VALUES (?, ?, ?)",
+    did,
+    method,
+    url,
+  );
+  return db.prepare(
+    "SELECT id, did, method, url FROM webhook_subscription WHERE id = last_insert_rowid()",
+  ).get<WebhookSubscriptionRow>()!;
+};
+
+export const deleteWebhooksByDid = (did: string): void => {
+  db.exec("DELETE FROM webhook_subscription WHERE did = ?", did);
+};
+
 export const upsertNote = (note: Note) => {
   const now = new Date().toISOString();
   db.exec(