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/api/types.go

@@ -29,6 +29,9 @@ type Zone struct {
 	// Registrar names the registrar Jarl can manage the domain at
 	// ("porkbun"), when one is configured for it.
 	Registrar string `json:"registrar,omitempty"`
+	// LinkedWith are the views of the zones this one is linked with
+	// (docs/views.md#linked-zones).
+	LinkedWith []string `json:"linked_with,omitempty"`
 	// Pending is set when listing with a changeset: add, change or delete.
 	Pending string `json:"pending,omitempty"`
 }
@@ -40,6 +43,8 @@ type Record struct {
 	View         string `json:"view,omitempty"`
 	RelativeName string `json:"relative_name"`
 	EffectiveTTL uint32 `json:"effective_ttl"`
+	// Linked is "shared" or "local" for a record of a linked zone ("" otherwise).
+	Linked string `json:"linked,omitempty"`
 	// Text is the decoded value of TXT and SPF records.
 	Text *string `json:"text,omitempty"`
 	// Pending is set when listing with a changeset: add, change, delete,