Disable records instead of deleting them

A disabled record stays in Jarl with its ID, TTL and comment but is
left out of the zone; the zone file lists it as a comment. Web UI
toggle, jarl record disable|enable, record.disable|enable operations
and API endpoints. Database schema 5.

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

internal/validate/records.go

@@ -103,6 +103,31 @@ func (c *checker) zoneLimit(z *model.Zone, name string, def int) int {
 	return n
 }
 
+// checkDisabledRecords warns about disabled records that couldn't be
+// enabled again as the zone is now: the CNAME rules are the usual way a
+// name changes while one of its records is disabled.
+func (c *checker) checkDisabledRecords(z *model.Zone) {
+	served := map[string]map[string]bool{}
+	for _, r := range c.zoneRecords[z.ID] {
+		if served[r.Name] == nil {
+			served[r.Name] = map[string]bool{}
+		}
+		served[r.Name][r.Type] = true
+	}
+	for _, r := range c.allRecords[z.ID] {
+		if !r.Disabled {
+			continue
+		}
+		types := served[r.Name]
+		switch {
+		case r.Type != "CNAME" && types["CNAME"]:
+			c.recProblem(z, r, Warning, "disabled_conflict", "the disabled record %s %s %s can't be enabled again while %s has a CNAME (a CNAME must be the only record at its name)", disp(r.Name), r.Type, r.Data, disp(r.Name))
+		case r.Type == "CNAME" && len(types) > 0:
+			c.recProblem(z, r, Warning, "disabled_conflict", "the disabled CNAME %s → %s can't be enabled again while %s has other records (a CNAME must be the only record at its name)", disp(r.Name), r.Data, disp(r.Name))
+		}
+	}
+}
+
 func (c *checker) checkZoneRecords(z *model.Zone) {
 	ix := &zoneIndex{
 		z:      z,