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/handlers/inline.rs

@@ -146,6 +146,15 @@ async fn answer_inline_query(bot: Bot, query: InlineQuery) -> Result<bool, Reque
             );
             for (i, media) in fetched.media.iter().enumerate() {
                 let id = format!("{i}");
+                // Telegram fetches an inline result's URL itself and cannot
+                // send site-specific headers, so hotlink-protected media
+                // (pixiv's pximg.net) would render as a broken file there.
+                // Locally produced media (ugoira MP4, bsky remux) is a local
+                // path and does not parse as a URL at all — same skip.
+                if x_media::site::needs_media_headers(media.url()) {
+                    log::debug!("inline: skipping hotlink-protected media {id}");
+                    continue;
+                }
                 let Some(url) = url::Url::parse(media.url()).ok() else {
                     continue;
                 };