Linked zones: shared records kept in step across views

Zones of the same name in different views can be linked. Shared records
are kept in step: a change in one view reaches the others in the same
changeset, as explicit operations, so the diff, permission checks,
history and undo cover every view. Records local to a view override the
shared records of that name and type there; removing the override brings
the shared ones back, and sharing a view's version makes it every view's.
Each zone keeps its own options, serial and file.

Linking existing zones changes no answer: what is the same everywhere
becomes shared, the rest stays local. Unlinking keeps every record, all
local. ACME challenges are written once per link, and --view all changes
a link once. jarl zone link/unlink, zone create --link-with, record
--local/--shared, LINK column, web UI controls, POST /zones/{zone}/link
and /unlink, validation checks linked_*, store migration 11, docs and
e2e.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

CHANGELOG.md

@@ -40,6 +40,20 @@ All notable changes to Jarl are documented here. The format follows
 
 ### Added
 
+- Linked zones ([docs/views.md](docs/views.md#linked-zones)): zones of the
+  same name in different views can be linked, and Jarl keeps their shared
+  records in step: a change in one view reaches the others in the same
+  changeset (diff, permissions, history and undo cover every view). Each
+  view can still have records of its own (`--local`, "Only here"), which
+  override the shared records of that name and type there, and keeps its
+  own zone options. Linking existing zones changes no answer: what is the
+  same everywhere becomes shared, the rest stays local, and a view's
+  version can be shared later (`--shared`, "Share"). `jarl zone link` and
+  `unlink`, `zone create --link-with VIEW`, `record add|set|update
+  --local|--shared`, a LINK column in `record list`, "Keep them linked"
+  when copying a zone in the web UI, `POST /zones/{zone}/link` and
+  `/unlink`. ACME challenges are written once per link, and `--view all`
+  changes a link once.
 - TLS certificates ([docs/certificates.md](docs/certificates.md)): Jarl
   obtains certificates from Let's Encrypt or any ACME CA (account set up
   in jarl.yaml under `acme.account`) and renews them itself, from the CA's