fix: repair four correctness defects in the send/state/handler paths

- send: the PHOTO_INVALID_DIMENSIONS marker never matched (the description is
  lower-cased, the marker was not), so oversized photos sent by URL were
  classified Permanent instead of taking the download-and-downscale fallback.
- inline: the debounce state was one global slot, so a second user's query
  cancelled the first user's pending answer entirely; it is now per user.
- state: prune_expired wrote back a stale snapshot without the per-chat lock,
  clobbering a concurrent update() (lost edit-message record -> "Expired");
  it now re-reads and prunes under the same lock update() uses.
- send/queue: a task dead-lettered on retry exhaustion kept its keep-alive
  temp media (ugoira MP4) alive until process exit; dead_letter_notify now
  releases it, and enqueue_retry releases when the enqueue itself fails.

Also folds the duplicated retry enqueue in post_send_actions into
send::enqueue_retry (single clock source, single place that releases).

Tests: +6 (marker, per-user debounce x3, keep-alive release, prune contract
x2); the marker and keep-alive cases were verified to fail before the fix.
cargo fmt/clippy clean, 52 + 69 tests pass.

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

@@ -1,7 +1,6 @@
 //! Callback query handling: the edit-before-forward prompt's "forward" and
 //! "template|<name>" buttons.
 
-use super::urls::enqueue_retry;
 use super::{CHAT_STORE, CONFIG, TASK_QUEUE};
 use crate::db::unix_now;
 use crate::send::{self, Task};
@@ -83,7 +82,7 @@ pub async fn callback_query_handler(bot: Bot, query: CallbackQuery) -> Result<()
                         task,
                     }) => {
                         log::info!("forward queued for retry in {delay_seconds:.1}s");
-                        enqueue_retry(&TASK_QUEUE, *task, delay_seconds).await;
+                        send::enqueue_retry(&TASK_QUEUE, *task, delay_seconds).await;
                         bot.answer_callback_query(callback_query_id)
                             .text("Forward queued for retry.")
                             .await?;