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/x-media/src/site/misskey/model.rs

@@ -0,0 +1,35 @@
+use serde::Deserialize;
+
+#[derive(Deserialize, Debug)]
+pub(crate) struct Note {
+    pub(crate) id: String,
+    pub(crate) text: Option<String>,
+    #[serde(default)]
+    pub(crate) cw: Option<String>,
+    pub(crate) user: User,
+    #[serde(default)]
+    pub(crate) files: Vec<DriveFile>,
+    /// Embedded original note when this note is a renote; the shell's own
+    /// text/files are usually empty and the content lives here.
+    #[serde(default)]
+    pub(crate) renote: Option<Box<Note>>,
+}
+
+#[derive(Deserialize, Debug)]
+pub(crate) struct User {
+    pub(crate) name: Option<String>,
+    pub(crate) username: String,
+}
+
+#[derive(Deserialize, Debug)]
+pub(crate) struct DriveFile {
+    #[serde(rename = "type")]
+    pub(crate) mime_type: String,
+    pub(crate) url: String,
+    #[serde(default, rename = "thumbnailUrl")]
+    pub(crate) thumbnail_url: Option<String>,
+    #[serde(default, rename = "isSensitive")]
+    pub(crate) is_sensitive: bool,
+    #[serde(default)]
+    pub(crate) name: Option<String>,
+}