Add admin-only case reset functionality

Introduce a new "reset" action for bulk operations and a dedicated
endpoint for individual case resets. These features allow administrators
to revert a case to its raw audio state by deleting derived artifacts
like transcripts, analysis input, and documents. The functionality also
includes renaming `.m4a.failed` files back to `.m4a` to re-trigger
transcription.

This commit also:
- Adds a `reset_case_artefacts` helper function to `case_actions.rs`.
- Implements the `handle_reset_case` endpoint.
- Adds the "Reset" button to the UI for administrators.
- Includes unit and integration tests to verify the new functionality
  and access control.

server/src/routes/user_web.rs

@@ -133,6 +133,9 @@ struct CaseDetailTemplate {
     /// per-recording "Transkription läuft…" label — suppresses the lie when
     /// a transcript is missing but no worker is active.
     transcribe_busy: bool,
+    /// True iff the session user is an admin — toggles admin-only controls
+    /// (currently the Reset button).
+    is_admin: bool,
 }
 
 /// Flags derived from filesystem state + config.
@@ -291,6 +294,7 @@ pub async fn handle_case_detail(
     let t_busy = transcribe_busy.0.load(Ordering::Acquire);
     let flags = compute_flags(&case_dir, &recordings, config.llm_configured(), a_busy).await;
     let case_id_short = case_id.chars().take(8).collect();
+    let is_admin = user.is_admin();
 
     CaseDetailTemplate {
         slug: user.slug,
@@ -303,6 +307,7 @@ pub async fn handle_case_detail(
         analyzing: flags.analyzing,
         has_document: flags.has_document,
         transcribe_busy: t_busy,
+        is_admin,
     }
     .render()
     .map(Html)