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/main.rs

@@ -8,6 +8,7 @@ use tokio::sync::watch;
 use x_media::site;
 
 mod config;
+mod ctx;
 mod db;
 mod handlers;
 mod link_cache;
@@ -18,6 +19,7 @@ mod rate_limit;
 mod send;
 mod state;
 
+use ctx::CONTEXT;
 use handlers::{CHAT_STORE, CONFIG, LINK_CACHE, TASK_QUEUE};
 
 /// Docker `stop` / `compose down` delivers SIGTERM, which teloxide's ctrlc
@@ -61,9 +63,13 @@ async fn main() {
     );
 
     // Queue worker: handles typed tasks, dead-letters failed sends to the
-    // task's chat.
+    // task's chat. Both closures use the shared context (the queue requires
+    // 'static handlers, and the statics are process-wide anyway).
     TASK_QUEUE
-        .start(send::handle_task, send::dead_letter_notify)
+        .start(
+            |payload| send::handle_task(&CONTEXT, payload),
+            |payload, message| send::dead_letter_notify(&CONTEXT, payload, message),
+        )
         .await;
     log::info!("task queue worker started");