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>

internal/model/model.go

@@ -166,6 +166,10 @@ type Zone struct {
 	// Disabled zones stay in Jarl, records and all, but named doesn't
 	// load them: their zone statement is left out of named.conf.
 	Disabled bool `json:"disabled,omitempty"`
+	// LinkGroup links zones of the same name in different views: Jarl
+	// keeps their shared records in step (docs/views.md#linked-zones). 0
+	// for a zone that isn't linked; linked zones share the ID.
+	LinkGroup int64 `json:"link_group,omitempty"`
 	Meta
 }
 
@@ -195,6 +199,10 @@ type Record struct {
 	// Disabled records stay in Jarl but are left out of the zone (the
 	// zone file lists them as comments).
 	Disabled bool `json:"disabled,omitempty"`
+	// SharedID ties the copies of a shared record in linked zones
+	// together (the same in every copy). 0 for a record of a zone that
+	// isn't linked, and for a record local to its view in one that is.
+	SharedID int64 `json:"shared_id,omitempty"`
 	Meta
 }