refactor(state): add ChatData::format_for for the per-site caption format

Four call sites (cache hit, fresh fetch, startup refetch, /debug preview)
spelled out the same message_format.get(...).cloned().unwrap_or_default();
the accessor names the lookup and keeps the empty-format contract in one
place.

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

@@ -626,10 +626,7 @@ pub(crate) async fn execute_command(
                     let format = CHAT_STORE
                         .get(message.chat.id.0)
                         .await
-                        .message_format
-                        .get(fetched.site_id)
-                        .cloned()
-                        .unwrap_or_default();
+                        .format_for(fetched.site_id);
                     let caption = preview_caption(
                         &format,
                         &fetched.caption,

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

@@ -638,11 +638,7 @@ async fn url_media_inner(
         // Cache keys are prefixed with the site id ("twitter:…"), matching
         // the value a fresh fetch would read from Fetched::site_id.
         let site = x_media::site::site_id_from_key(&key);
-        let format = chat_data
-            .message_format
-            .get(site)
-            .cloned()
-            .unwrap_or_default();
+        let format = chat_data.format_for(site);
         // One call for both: `caption_from_fields` returns the truncated
         // built-in caption itself when the chat has no format for this site.
         let caption = x_media::site::caption_from_fields(
@@ -713,11 +709,7 @@ async fn url_media_inner(
             }
             let chat_data = ctx.chat_store.get(chat_id).await;
             // Per-site caption format override (empty -> built-in caption).
-            let format = chat_data
-                .message_format
-                .get(fetched.site_id)
-                .cloned()
-                .unwrap_or_default();
+            let format = chat_data.format_for(fetched.site_id);
             let caption = fetched.caption_with(&format);
             // Raw render data for the link cache; the send fills in the
             // Telegram file ids and persists the entry.
@@ -862,11 +854,7 @@ async fn refetch(
         return Ok(None);
     }
     let chat_data = ctx.chat_store.get(chat_id).await;
-    let format = chat_data
-        .message_format
-        .get(fetched.site_id)
-        .cloned()
-        .unwrap_or_default();
+    let format = chat_data.format_for(fetched.site_id);
     let caption = fetched.caption_with(&format);
     let cache_data = fetched
         .render_fields()