Rewrite docs/files.md to the v5 API: single UploadEarlyEndpoint with the
upload-then-reference flow, jail/ready scanner quarantine, ServerFile in
models, signed URLs, and the one-instance-per-server footgun. Replace the
v4 HttpContent/FileObject examples with the TypedData/PublicFileSystem API
and drop the removed Azure backend.

Replace the docs/websockets.md TODO stub with real docs: WebSocketHandler
lifecycle, pub/sub topics, MultiplexWebSocketHandler, typed
ApiWebsocketHandler, and local vs AWS execution models.

Add docs/migration-v4-to-v5.md covering the service-abstractions
extraction, ServerBuilder / definition-vs-runtime split, dependency and
import moves, and common pitfalls, deferring KiteUI client details to the
ls5-kui7-migration skill.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
(cherry picked from commit 3c6c27e)
(cherry picked from commit 5bd3d6f)