Refactor busy flags and worker guards

The `WorkerBusy` type was previously a single `Arc<AtomicBool>` shared
between the analyze and transcribe workers. This commit refactors this
to:

- Introduce `AnalyzeBusy` and `TranscribeBusy` newtype wrappers around
  `WorkerBusy` to distinguish between the two flags. This allows
  `axum::extract::State` to target them individually.
- Move the `BusyGuard` RAII guard into `src/lib.rs` and make it generic
  to work with any `WorkerBusy` instance.
- Update the `main.rs`, `worker.rs`, and `routes/user_web.rs` files to
  use the new types and guards, ensuring correct state management for
  both pipelines.
- Enhance the transcription recovery scan
  (`transcribe::recovery::scan_and_enqueue`) to iterate over user
  directories and call `enqueue_pending_for_user` for each, making it
  more robust and aligned with the per-user self-healing mechanism.
- Add a check for `transcribe_busy` in the `case_detail.html` template
  to correctly display "Transkription läuft…" only when the transcribe
  worker is actually active.

server/tests/transcribe_test.rs

@@ -286,7 +286,8 @@ async fn worker_renames_audio_to_failed_on_whisper_error() {
     drop(tx); // close channel so the worker loop exits after processing the job
 
     let client = reqwest::Client::new();
-    transcribe::worker::run(rx, config, client).await;
+    let busy = std::sync::Arc::new(std::sync::atomic::AtomicBool::new(false));
+    transcribe::worker::run(rx, config, client, busy).await;
 
     // Audio must have been renamed so recovery skips it next time.
     assert!(!audio.exists(), "original .m4a still present");