fix(auth): issue a Secure session cookie under the __Host- prefix

A cookie named healthlog_session can be set by any host under the same
registrable domain, so a page on a sibling subdomain could plant a session of
its choosing in a visitor's browser. Wherever the session cookie is Secure it
is now issued as __Host-healthlog_session, which a browser accepts only from
this exact host. Plain-HTTP installs, which cannot hold a __Host- cookie,
keep the old name.

Nobody is signed out: the old name is still read after the new one, a
session found under it is re-issued under the new name and the old cookie
expired, and a browser holding the new cookie is never answered from the old
one. Every reader and writer goes through one module, which the proxy, the
native-caller check, the native handoff and the OIDC callback use; the
session-surface guard follows the new reader.

docs/api/openapi.yaml

@@ -94543,4 +94543,6 @@ components:
       type: apiKey
       in: cookie
       name: healthlog_session
-      description: Browser session cookie — set by /api/auth/login.
+      description: Browser session cookie — set by /api/auth/login. Named `__Host-healthlog_session` wherever the cookie is
+        `Secure` (every HTTPS install); a plain-HTTP install, which cannot hold a `__Host-` cookie, keeps
+        `healthlog_session`.