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/src/transcribe/worker.rs

@@ -6,16 +6,23 @@ use tracing::{error, info, warn};
 
 use super::{ffmpeg, ollama, whisper, TranscribeReceiver};
 use crate::config::{Config, WhisperUserSettings};
+use crate::{BusyGuard, WorkerBusy};
 
 const ONELINER_TIMEOUT: Duration = Duration::from_secs(60);
 
 /// Consume transcription jobs sequentially. Whisper holds the GPU, so parallelism
 /// here would only cause contention. One job in flight at a time.
-pub async fn run(mut rx: TranscribeReceiver, config: Arc<Config>, client: reqwest::Client) {
+pub async fn run(
+    mut rx: TranscribeReceiver,
+    config: Arc<Config>,
+    client: reqwest::Client,
+    worker_busy: WorkerBusy,
+) {
     info!("Transcription worker started");
     let timeout = Duration::from_secs(config.whisper_timeout_seconds);
 
     while let Some(job) = rx.recv().await {
+        let _guard = BusyGuard::new(worker_busy.clone());
         let audio_path = job.audio_path;
         let transcript_path = audio_path.with_extension("transcript.txt");