refactor(handlers): split urls.rs into workers, pipeline and startup repair

urls.rs carried five reasons to change: the job channel and its worker pool,
the single-flight fetch, URL parsing, the per-URL pipeline, and the startup
repair of queued retries. The two with their own lifecycle move out:

- url_workers.rs: the bounded channel, its supervised pool and
  start/stop_url_workers (the pipeline stays in urls.rs, which the workers
  call).
- repair.rs: needs_refetch/apply_refresh/refetch/repair_lost_local_media with
  their tests, moved whole (the live one keeps its #[ignore]).

Also folds the two byte-identical render_fields -> CachedPost mappings
(urls.rs and repair.rs) into urls::cached_snapshot, and moves the shared
permanent_error test fixture into ctx::test_support.

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

@@ -9,18 +9,20 @@
 mod callback;
 mod commands;
 mod inline;
+mod repair;
 mod statics;
+mod url_workers;
 mod urls;
 
 pub use callback::callback_query_handler;
 pub use commands::register_commands;
 pub use inline::inline_query_handler;
 pub(crate) use inline::prune_idle_states;
+pub(crate) use repair::repair_lost_local_media;
 /// The resolved `$DATA_DIR/task_queue.db` path, for the startup config line.
 pub(crate) use statics::db_path;
 pub use statics::{CHAT_STORE, CONFIG, LINK_CACHE, TASK_QUEUE};
-pub(crate) use urls::repair_lost_local_media;
-pub use urls::{start_url_workers, stop_url_workers};
+pub use url_workers::{start_url_workers, stop_url_workers};
 
 use crate::ctx::AppContext;
 use crate::media_sender::MediaSender;
@@ -29,7 +31,8 @@ use teloxide::RequestError;
 use teloxide::prelude::*;
 use teloxide::types::{ChatId, Message, MessageId};
 use teloxide::utils::command::BotCommands;
-use urls::{URL_JOBS, extract_urls};
+use url_workers::URL_JOBS;
+use urls::extract_urls;
 
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