Send altbody so campaigns actually go out multipart/alternative

listmonk never derives a plaintext part from HTML/Markdown on its own
(models/campaigns.go only compiles AltBodyTpl, and
internal/manager/message.go only emits text/plain, when altbody is
explicitly non-null) -- confirmed against the deployed v6.2.0 source.
Every campaign this tool created was HTML-only as a result, contrary
to the README's now-corrected claim.

Reusing the same raw Markdown+template source as altbody personalizes
it identically to body, since AltBodyTpl compiles whenever the text
contains {{ }} expressions.

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

internal/listmonk/listmonk.go

@@ -335,7 +335,17 @@ func (in CampaignInput) payload() map[string]any {
 		"lists":        in.ListIDs,
 		"content_type": "markdown",
 		"body":         in.Body,
-		"type":         in.Type,
+		// listmonk never derives a plaintext alternative from body/HTML on its
+		// own (models/campaigns.go only compiles AltBodyTpl, and
+		// internal/manager/message.go only emits a text/plain part, when
+		// altbody is explicitly non-null) — confirmed against the deployed
+		// v6.2.0 source. Reusing the same raw Markdown+template source as the
+		// altbody gets it personalized identically to body (AltBodyTpl compiles
+		// whenever the text contains {{ }} expressions), at the cost of
+		// Markdown syntax like *emphasis* showing up literally in plaintext
+		// clients — an acceptable, well-established tradeoff for Markdown.
+		"altbody": 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