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/x-media/src/site/twitter/interface.rs

@@ -43,27 +43,25 @@ pub async fn fetch_from_url(url: &str) -> Result<Fetched, FetchError> {
     match fetch(id).await {
         Ok(tweet) => Ok(tweet.into()),
         // Syndication withholds NSFW/age-restricted tweets (empty `{}`).
-        // Retry as the logged-in user when TWITTER_AUTH_TOKEN is set;
-        // otherwise degrade to an empty result (the bot replies
-        // "No media found").
+        // Retry as the logged-in user when TWITTER_AUTH_TOKEN is set; without
+        // the token the withholding is reported as `Sensitive`, so the bot can
+        // answer "age-restricted / needs TWITTER_AUTH_TOKEN" instead of the
+        // misleading "No media found".
         Err(FetchError::Sensitive) => {
             if super::auth::enabled() {
                 match super::auth::fetch(id).await {
                     Ok(tweet) => Ok(tweet.into()),
-                    // The tweet is genuinely gone (deleted / suspended /
-                    // tombstoned): report it instead of degrading to an
-                    // empty result ("No media found"). Only unexpected
-                    // fallback failures (network, parse) keep the NSFW
-                    // placeholder.
-                    Err(FetchError::NotFound) => Err(FetchError::NotFound),
+                    // Deleted/suspended (tombstoned) and unexpected fallback
+                    // failures keep their own class: the bot reports what
+                    // actually happened rather than "No media found".
                     Err(e) => {
                         log::warn!("twitter auth fallback failed for {id}: {e}");
-                        Ok(empty_fetched(url))
+                        Err(e)
                     }
                 }
             } else {
                 log::debug!("tweet {id} is sensitive; set TWITTER_AUTH_TOKEN to fetch NSFW media");
-                Ok(empty_fetched(url))
+                Err(FetchError::Sensitive)
             }
         }
         Err(e) => Err(e),
@@ -90,24 +88,6 @@ pub fn media_headers(_url: &str) -> Option<Vec<(&'static str, String)>> {
     None
 }
 
-/// A Fetched with no media for withheld tweets: the bot replies
-/// "No media found" and moves on instead of erroring.
-fn empty_fetched(url: &str) -> Fetched {
-    Fetched {
-        source_url: url.to_string(),
-        // The raw user-supplied URL goes into an HTML caption; escape it so
-        // crafted links cannot break the parse (Telegram 400).
-        caption: encode_text(url).into_owned(),
-        title: String::new(),
-        content: String::new(),
-        media: vec![],
-        sensitive: true,
-        site_id: "twitter",
-        render_data: None,
-        _keep_alive: None,
-    }
-}
-
 /// Fetches a tweet from the syndication endpoint. Deleted/blocked tweets
 /// surface as `FetchError::NotFound`; withheld content (empty tombstone,
 /// age-restricted) as `FetchError::Sensitive`.