refactor: finish the phase-B seam for the post-send path, funnel settlement

docs/architecture-refactor.md §3 stopped half-done: `url_media` got an injected
`AppContext`, but `send.rs`'s post-send half kept reaching for the process-wide
`CHAT_STORE`/`TASK_QUEUE`/`LINK_CACHE` statics, so the whole shell after a
successful send (edit-before-forward prompt, channel forward, retry enqueue,
cache write) had no test and no way to get one.

- `ctx.rs` now owns `AppContext` (sender + the three stores + config) with
  `from_statics` for production and a `CONTEXT` static for the spawned worker
  closures; `handlers/urls.rs` drops its private copy and the duplicated
  assembler, and the queue handler/dead-letter callbacks take the context
  (main wires them with `CONTEXT`).
- `send_media_sequence`/`send_animation`/`forward_messages`/`post_send_actions`
  take `&AppContext`; the cache write goes through the injected cache.
- New `settle_task(ctx, task, Sent|Failed)` is the single place that ends a
  task: release its keep-alive temp media, and drop the link-cache entry only
  on failure. All five former call sites funnel through it — the earlier
  keep-alive leak existed precisely because one of them had to remember.
  `invalidate_cache`/`invalidate_cache_with` (static + injected pair, the
  latter only existing because of the former) collapse into one private fn.
- `ctx::test_support::TestStores` gives tests a tempdir store set + context;
  `handlers/urls.rs` tests use it instead of hand-rolled setup.

Tests: +5 (post-send forward ok / queued / notified, settle Sent/Failed); the
post-send and settle paths were previously untested. fmt/clippy clean,
60 + 69 tests pass.

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

@@ -50,6 +50,7 @@ pub async fn callback_query_handler(bot: Bot, query: CallbackQuery) -> Result<()
         chat_id
     );
     if data == "forward" {
+        let ctx = crate::ctx::AppContext::from_statics(&bot);
         match chat_data.forward_channel_id {
             Some(channel_id) => {
                 let forward_task = Task::ForwardMessages {
@@ -59,7 +60,7 @@ pub async fn callback_query_handler(bot: Bot, query: CallbackQuery) -> Result<()
                     notify_chat_id: Some(chat_id),
                     notify_message_id: Some(prompt_message_id),
                 };
-                match send::forward_messages(&bot, &forward_task).await {
+                match send::forward_messages(&ctx, &forward_task).await {
                     Ok(()) => {
                         log::info!(
                             "forwarded {} message(s) to channel {channel_id}",