chore(release): v1.38.20 — signing in no longer says who has an account here

Bump the version, the service-worker fallback and the OpenAPI document,
and add the CHANGELOG entry for v1.38.20.

docs/api/openapi.yaml

@@ -1,7 +1,7 @@
 openapi: 3.1.0
 info:
   title: HealthLog API
-  version: 1.38.19
+  version: 1.38.20
   description: >-
     Self-hosted personal-health-tracking PWA — public API surface for the iOS native client and external ingest.