Always send messenger=email in campaign payloads

Discovered while verifying the TestCampaign fix against real listmonk:
create/update apparently default an omitted messenger to "email"
server-side, but the test-send endpoint validates the raw request body
without that same defaulting, failing with "Unknown messenger .".
eec-campaigns only ever targets email, so send it explicitly instead
of depending on an inconsistency between endpoints.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

internal/listmonk/listmonk.go

@@ -336,6 +336,12 @@ func (in CampaignInput) payload() map[string]any {
 		"content_type": "markdown",
 		"body":         in.Body,
 		"type":         in.Type,
+		// listmonk's create/update handlers default an omitted messenger to
+		// "email" before validating, but the test-send handler validates the
+		// raw request body as-is — an omitted messenger there fails with
+		// "Unknown messenger .". eec-campaigns only ever sends email, so set
+		// it explicitly everywhere rather than relying on that asymmetry.
+		"messenger": "email",
 	}
 	if in.FromEmail != "" {
 		p["from_email"] = in.FromEmail