Fix spurious re-sync/preview for campaigns without an explicit template_id

campaignUnchanged compared input.TemplateID (0 when unset) literally
against existing.TemplateID, but listmonk always assigns a real, nonzero
template_id server-side even when create/update omits it. Every campaign
without an explicit template_id was therefore flagged "changed" on every
sync, forever, firing an unwanted extra preview email each time — caught
via reground-campaigns' welcome/campaign.md re-previewing on an unrelated
push. Treat TemplateID 0 as "don't compare" instead. Test fake now mirrors
listmonk's real default-assignment behavior so this class of bug is caught.

internal/campaign/sync.go

@@ -222,7 +222,13 @@ func campaignUnchanged(existing *listmonk.Campaign, input listmonk.CampaignInput
 		// stored altbody is still null even though its body hasn't changed.
 		existing.AltBody == input.Body &&
 		existing.FromEmail == input.FromEmail &&
-		existing.TemplateID == input.TemplateID &&
+		// input.TemplateID == 0 means "unspecified, use whatever's assigned"
+		// (see CampaignInput.TemplateID) -- listmonk always assigns some real,
+		// nonzero template_id server-side even when it's omitted from the
+		// create/update payload, so comparing 0 against that would flag every
+		// campaign.md without an explicit template_id as changed on every
+		// sync, forever.
+		(input.TemplateID == 0 || existing.TemplateID == input.TemplateID) &&
 		existing.Type == input.Type &&
 		equalStringSets(existing.Tags, input.Tags) &&
 		equalIntSets(existing.ListIDs, input.ListIDs) &&