chore(release): v1.38.19 — the setup says what is true, and the AI has budget left when you ask

Bump the version, the service-worker fallback and the OpenAPI document,
and add the CHANGELOG entry for v1.38.19.

docs/api/openapi.yaml

@@ -1,7 +1,7 @@
 openapi: 3.1.0
 info:
   title: HealthLog API
-  version: 1.38.18
+  version: 1.38.19
   description: >-
     Self-hosted personal-health-tracking PWA — public API surface for the iOS native client and external ingest.