The webhook CRUD test combined an explicit did:plc:user webhook
with seedWebhook calls that defaulted to did:plc:test, so
listWebhooksByDid("did:plc:user") only returned the one
explicit row instead of three.
withTestDb sets up an in-memory SQLite handle with migrations
applied, swaps the production singleton via _setDbForTest, and
restores in a finally block. installFetchStub captures every
outbound request and throws if restored twice. requestApp wraps
Oak's app.handle so route tests can run without listen().
stubAuthenticate flips the server's authenticator indirection.
