feat(ux): onboard users, expose the chat's settings, name failed posts

`/start` was "Hello!" and `/help` was the bare command list teloxide can
render — no argument syntax, no caption placeholders, no mention that
links only work in private chats. Both now carry that guidance, and the
bot's profile description / short description are set at startup so a
shared link says what the bot does.

`/settings` reports what this chat is configured to do (forward channel,
edit-before-forward, per-site formats, saved templates) to anyone in the
chat — `/bot_dict` is a raw admin-only dump. Templates can be removed
(`/remove_template`, listing the live names on a typo) and the prompt's
keyboard folds 3 per row with a cap: Telegram rejects a keyboard over 100
buttons outright, which would silently drop the whole prompt.

Inline results hand URLs to Telegram, which fetches them without any
site headers — pixiv's pximg.net answers 403 to that, so those items are
skipped instead of shipped broken. `needs_media_headers` answers that
question from the same per-site rule the downloader uses.

Dead-letter and retry notices name the failing post and the cause
(`failure_text`), since "Task failed after retries: task failed after 2
retries" said neither which link it was nor what happened.

crates/xmedia-bot/src/queue.rs

@@ -342,8 +342,16 @@ impl QueueWorker {
                 payload,
             }) => {
                 if row.attempts as u32 >= MAX_RETRIES {
-                    let message = format!("task failed after {MAX_RETRIES} retries");
-                    log::error!("dead-lettering {}: {message}", row.id);
+                    // The queue keeps only the payload, not the last error, so
+                    // the cause of an exhausted retry is just that: exhausted.
+                    // (The dead-letter message is read by the user, so it must
+                    // not restate its own wrapper — see `failure_text`.)
+                    let message = "retries exhausted".to_string();
+                    log::error!(
+                        "dead-lettering {}: {message} after {} attempt(s)",
+                        row.id,
+                        row.attempts + 1
+                    );
                     self.delete_row(&row.id).await;
                     (self.dead_letter)(payload, message).await;
                 } else {