apt: move the keyserver lookup URL to data/keyserver.yml

data/keyserver.yml

@@ -0,0 +1,17 @@
+## Keyserver lookup endpoint used to fetch PPA signing keys.
+## Like host_keys.yml, this file exists so that a static endpoint is data,
+## updatable in one reviewable place, instead of hardcoded in the source —
+## the URL was previously duplicated in the apt keyring and release
+## modules. Sparse on purpose: it grows if keyserver pools or alternates
+## ever need to be tried.
+##
+## The template carries its variable part as a {fingerprint} placeholder,
+## substituted by the accessor of src/apt/keyring.rs with plain string
+## replacement.
+##
+## Where the value comes from: keyserver.ubuntu.com, Ubuntu's OpenPKS
+## (formerly SKS) keyserver; op=get with search=0x<fingerprint> is the
+## documented machine interface fetching one key by fingerprint
+## (https://keyserver.ubuntu.com).
+
+lookup_template: "https://keyserver.ubuntu.com/pks/lookup?op=get&search=0x{fingerprint}"