feat(send): quote a long post's text in an expandable blockquote

A post whose text (the split `title` plus `content`, joined by
`site::compose_text`) reaches `CAPTION_QUOTE_TEXT_CHARS` — default 200,
`0` disables — now has that text wrapped in `<blockquote expandable>`
inside its caption, leaving the URL and author line outside the quote.

Applied at the send boundary (`send_media_sequence`, `send_animation` and
the inline answers), where the caption is already truncated and the same
cache snapshot supplies the text, so a fresh send, a link-cache resend
and a queued retry all decide identically. The text is located as what
follows the author link, with the visible prefix accepted as a match
because `truncate_caption` may cut inside it — that keeps the longest
posts, the ones that most need folding, quoted. Captions whose layout
moves the text elsewhere (pixiv's title-inside-a-link, a `/set_format`
that puts `{title}`/`{content}` first) stay unquoted rather than risking
a blockquote nested in a tag, and a caption that already carries one is
never wrapped again.

Telegram measures a caption *after entities parsing*, so the tags cost no
length and the 1024-character limit cannot be breached; retries replay
the unwrapped caption, so a threshold change takes effect immediately.
The edit-before-forward rewrite stays unquoted by design.

Verified against Telegram: a media-group caption built this way comes
back with `caption_entities` `url` @0, `text_link` @50,
`expandable_blockquote` @56 — the quote starts after the author line.

crates/xmedia-bot/src/config.rs

@@ -13,6 +13,10 @@ pub struct Config {
     pub edit_message_ttl: Duration,
     /// LINK_CACHE_TTL_SECONDS, default 604800 (7 days).
     pub link_cache_ttl: Duration,
+    /// CAPTION_QUOTE_TEXT_CHARS, default 200: a post whose text (title plus
+    /// content) is at least this many characters gets that text wrapped in an
+    /// expandable blockquote inside its caption. `0` disables the wrap.
+    pub caption_quote_text_chars: usize,
     // Webhook settings (moved out of main; names/defaults unchanged).
     pub webhook_enabled: bool,
     pub webhook_url: Option<url::Url>,
@@ -58,6 +62,7 @@ impl Config {
             Duration::from_secs(parse_u64("EDIT_MESSAGE_TTL_SECONDS", 24 * 3600));
         let link_cache_ttl =
             Duration::from_secs(parse_u64("LINK_CACHE_TTL_SECONDS", 7 * 24 * 3600));
+        let caption_quote_text_chars = parse_u64("CAPTION_QUOTE_TEXT_CHARS", 200) as usize;
 
         let webhook_enabled = env::var("WEBHOOK")
             .is_ok_and(|v| matches!(v.to_lowercase().as_str(), "true" | "yes" | "1"));
@@ -93,6 +98,7 @@ impl Config {
             admin_ids,
             edit_message_ttl,
             link_cache_ttl,
+            caption_quote_text_chars,
             webhook_enabled,
             webhook_url,
             webhook_listen,