feat(ux): answer every link, name fetch failures, keep the chat action alive

Four ways a user could get silence are closed: a registered-but-disabled
site (pixiv without a token) now answers instead of being dropped as an
unsupported link, `/test` on such a link replies instead of doing nothing,
a supported link posted in a group gets a one-line hint (channels stay
silent), and fetch failures name their cause — gone / withheld / source
risk control / site disabled / source down — instead of one generic
sentence. `FetchError::Disabled` carries the "matched but switched off"
answer, which `find_site` used to fold into `Ok(None)`.

A withheld tweet no longer degrades to "no media": without
`TWITTER_AUTH_TOKEN` it stays `Sensitive` so the reply says the media is
age-restricted, and a failed authenticated fallback propagates its own
class instead of masquerading as an empty post (`empty_fetched` is gone).

Long jobs stop looking stalled: `run_with_chat_action` re-sends the chat
action every 4s while the pipeline is pending and the hint switches from
typing to send-photo/video once the media kinds are known. Media groups
go from 9 to Telegram's 10.

`/set_format` rejects unknown `{…}` placeholders (a typo used to be
published verbatim in every caption) and resets with `-`. The
edit-before-forward prompt states its TTL and that Confirm is required,
gains a Skip button, and is rewritten in place to "expired" by the sweep
— an edit, never a new message, so a background timer cannot wake a chat.

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

@@ -14,6 +14,8 @@ use teloxide::types::{CallbackQuery, CallbackQueryId, MessageId};
 
 /// The `"forward"` button's data.
 const FORWARD: &str = "forward";
+/// The `"skip"` button's data: drop the prompt without forwarding.
+const SKIP: &str = "skip";
 /// Prefix of a template button's data: `"template|<name>"`.
 const TEMPLATE_PREFIX: &str = "template|";
 
@@ -70,6 +72,29 @@ async fn handle_callback(
     }
 
     log::info!("callback from {chat_id} on prompt {prompt_message_id}: {data}");
+    if data == SKIP {
+        // Skip works with or without a forward channel: it is the explicit
+        // "do not forward this" answer, and it drops the record so the forward
+        // can never happen later.
+        log::info!("edit-before-forward prompt {prompt_message_id} skipped");
+        ctx.chat_store
+            .update(chat_id, |data| {
+                data.edit_message.remove(&prompt_message_id);
+            })
+            .await;
+        let _ = ctx
+            .sender
+            .delete_message(ChatId(chat_id), MessageId(prompt_message_id as i32))
+            .await;
+        let _ = ctx
+            .sender
+            .answer_callback_query(
+                callback_query_id,
+                Some("Skipped — nothing was forwarded.".to_string()),
+            )
+            .await;
+        return;
+    }
     if data == FORWARD {
         match chat_data.forward_channel_id {
             Some(channel_id) => {
@@ -244,6 +269,31 @@ mod tests {
         );
     }
 
+    #[tokio::test]
+    async fn skip_drops_the_prompt_without_forwarding() {
+        // "skip" needs no forward channel and no scripted outcomes: it deletes
+        // the prompt and drops the record, so no forward can ever happen.
+        let sender = MockSender::scripted(vec![], api_error);
+        let stores = TestStores::new();
+        let ctx = stores.ctx(&sender);
+        seed_prompt(&ctx, crate::db::unix_now()).await;
+
+        handle_callback(&ctx, callback_id(), 1, PROMPT_ID, "skip").await;
+
+        assert_eq!(
+            sender.calls(),
+            vec!["delete_message", "answer_callback_query"]
+        );
+        assert_eq!(
+            sender.answers(),
+            vec![Some("Skipped — nothing was forwarded.".to_string())]
+        );
+        assert!(
+            ctx.chat_store.get(1).await.edit_message.is_empty(),
+            "a skipped prompt must drop its record"
+        );
+    }
+
     #[tokio::test]
     async fn forward_without_a_channel_is_reported() {
         let sender = MockSender::scripted(vec![], api_error);