fix: deliver alt texts to providers at publish time

Intake stores alt texts positionally (aligned with mediaAssetIds) but the
Mastodon upload reads an asset-id-keyed providerMetadata.altText map, so
descriptions were silently dropped. postOf translates the array into the map.

apps/scheduler/src/activities/publish.activity.ts

@@ -23,7 +23,20 @@ function connectionOf(job: ClaimedJob): ConnectorConnection {
 function postOf(job: ClaimedJob): NormalizedPost {
   const visibility = job.post.visibility;
   if (!["public", "unlisted", "private", "followers", "provider_default"].includes(visibility)) throw new Error("INVALID_POST_VISIBILITY");
-  return { ...job.post, visibility: visibility as NormalizedPost["visibility"] };
+  const post = job.post;
+  // Positional alt texts (aligned with mediaAssetIds at intake) become the
+  // connector's asset-id → description map; the Mastodon upload reads it.
+  const providerMetadata: Record<string, unknown> = { ...post.providerMetadata };
+  const altTexts = post.altTexts;
+  if (Array.isArray(altTexts) && altTexts.length > 0) {
+    const altText: Record<string, string> = {};
+    post.mediaAssetIds.forEach((id, index) => {
+      const text = altTexts[index];
+      if (typeof text === "string" && text.trim() !== "") altText[id] = text;
+    });
+    if (Object.keys(altText).length > 0) providerMetadata.altText = altText;
+  }
+  return { ...post, visibility: visibility as NormalizedPost["visibility"], providerMetadata };
 }
 /** Persist only documented remote evidence; unknown fields never enter DB/history. */
 export function safeMetadata(metadata: Record<string, unknown>): Record<string, unknown> {