Pin the UTC offset on the time a flow version carries

Every datetime the API emits for versions (flow summaries' updated_at,
runs' created/started/finished with flow_version) is already ISO 8601
with Z or +00:00. A test holds it and the API docs say so.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WgBfkm4rCmtd1T4vHWjEPx

docs/code/api.md

@@ -12,7 +12,8 @@ export FLUKSIO=http://127.0.0.1:8000/api/v1
 Use the port printed by `fluksio serve`. For Docker, use
 `http://api.localhost/api/v1`, `http://<host-ip>:8080/api/v1` on a LAN stack,
 or `https://api.<your-domain>/api/v1` for a deployment.
-All paths below are relative to this base.
+All paths below are relative to this base. Timestamps in responses are
+ISO 8601 and always carry their UTC offset.
 
 ## Authenticating