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>

README.md

@@ -54,8 +54,11 @@ attachments:
 ---
 
 Campaign body goes here, in Markdown. It's sent to listmonk with
-content_type "markdown" — listmonk renders the HTML *and* derives the
-plaintext alternative itself.
+content_type "markdown" for the HTML part; listmonk never derives a
+plaintext alternative on its own, so this same source is also sent
+verbatim as `altbody` — recipients on plaintext-only clients see the
+raw Markdown (and personalization still resolves, since the `{{ }}`
+expressions are still there), rather than getting an HTML-only email.
 ```
 
 - **`subject`** — required.