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/analyze/worker.rs

@@ -1,5 +1,4 @@
 use std::path::Path;
-use std::sync::atomic::Ordering;
 use std::sync::Arc;
 use std::time::Duration;
 
@@ -8,25 +7,7 @@ use tracing::{error, info, warn};
 
 use super::{llm, prompt, AnalysisInput, AnalyzeReceiver};
 use crate::config::Config;
-use crate::WorkerBusy;
-
-/// RAII guard that flips `WorkerBusy` to `true` on construction and back to
-/// `false` on drop. Panic-safe: even if `process()` unwinds, the flag is
-/// cleared.
-struct BusyGuard(WorkerBusy);
-
-impl BusyGuard {
-    fn new(busy: WorkerBusy) -> Self {
-        busy.store(true, Ordering::Release);
-        Self(busy)
-    }
-}
-
-impl Drop for BusyGuard {
-    fn drop(&mut self) {
-        self.0.store(false, Ordering::Release);
-    }
-}
+use crate::{BusyGuard, WorkerBusy};
 
 /// Consume analyze jobs sequentially. The external LLM tolerates parallelism,
 /// but sequential processing keeps the architecture simple and shields the