feat: add Misskey (misskey.io) fetch support

Fourth site adapter: POST /api/notes/show, renote-aware caption and
media normalization, DriveFile type → Illustration/Animated/Video.
Empty thumbnailUrl strings filtered out in thumbnail_for.

crates/xmedia-bot/src/handlers/commands.rs

@@ -253,7 +253,7 @@ pub(crate) async fn execute_command(
                     bot,
                     message.chat.id.0,
                     message.id,
-                    "Unknown site. Use twitter, bsky or pixiv.",
+                    "Unknown site. Use twitter, bsky, pixiv or misskey.",
                 )
                 .await?;
                 return Ok(());
@@ -294,7 +294,7 @@ pub(crate) async fn execute_command(
                             bot,
                             message.chat.id.0,
                             message.id,
-                            "Unrecognized link. Use a twitter/x, pixiv or bsky post URL.",
+                            "Unrecognized link. Use a twitter/x, pixiv, bsky or misskey post URL.",
                         )
                         .await?;
                         return Ok(());
@@ -337,7 +337,7 @@ pub(crate) async fn execute_command(
                         bot,
                         message.chat.id.0,
                         message.id,
-                        "No enabled site matches this link (twitter/x, pixiv or bsky).",
+                        "No enabled site matches this link (twitter/x, pixiv, bsky or misskey).",
                     )
                     .await?;
                 }

crates/xmedia-bot/src/handlers/urls.rs

@@ -135,7 +135,12 @@ pub fn extract_urls(message: &Message) -> Vec<String> {
 fn thumbnail_for(media: &Media) -> Option<String> {
     let url = media.url();
     if url.starts_with("http://") || url.starts_with("https://") {
-        media.thumbnail_url().map(str::to_string)
+        // An empty thumbnail string (misskey video/gif files without a
+        // thumbnailUrl) must not reach Telegram; let it generate its own.
+        media
+            .thumbnail_url()
+            .map(str::to_string)
+            .filter(|t| !t.is_empty())
     } else {
         None
     }