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/bulk.rs

@@ -13,7 +13,7 @@ use crate::auth::AuthenticatedWebUser;
 use crate::config::Config;
 use crate::error::AppError;
 use crate::paths::{write_delete_marker, DeleteMarker};
-use crate::routes::case_actions::{build_analysis_input, write_input_create_new};
+use crate::routes::case_actions::{build_analysis_input, reset_case_artefacts, write_input_create_new};
 use crate::routes::user_web::locate_case;
 
 #[derive(Debug, Deserialize)]
@@ -39,6 +39,13 @@ pub async fn handle_bulk_action(
     match form.action.as_str() {
         "analyze" => bulk_analyze(&config, &analyze_tx, &user.slug, &user_root, &form.case_ids).await,
         "delete" => bulk_delete(&user.slug, &user_root, &form.case_ids).await,
+        "reset" => {
+            if !user.is_admin() {
+                warn!(slug = %user.slug, "bulk-reset: not admin");
+                return Err(AppError::Forbidden("Nur für Admins".into()));
+            }
+            bulk_reset(&user.slug, &user_root, &form.case_ids).await
+        }
         other => {
             warn!(slug = %user.slug, action = %other, "bulk: unknown action");
             return Err(AppError::BadRequest("Unbekannte Aktion".into()));
@@ -148,3 +155,27 @@ async fn bulk_delete(slug: &str, user_root: &std::path::Path, case_ids: &[String
     }
     info!(slug = %slug, batch = %batch, ok, skipped, "bulk-delete completed");
 }
+
+async fn bulk_reset(slug: &str, user_root: &std::path::Path, case_ids: &[String]) {
+    let mut ok = 0usize;
+    let mut skipped = 0usize;
+    for case_id in case_ids {
+        if uuid::Uuid::parse_str(case_id).is_err() {
+            warn!(slug = %slug, case_id = %case_id, "bulk-reset: invalid case_id");
+            skipped += 1;
+            continue;
+        }
+        let Some(case_dir) = locate_case(user_root, case_id).await else {
+            warn!(slug = %slug, case_id = %case_id, "bulk-reset: case not found");
+            skipped += 1;
+            continue;
+        };
+        if let Err(e) = reset_case_artefacts(&case_dir).await {
+            warn!(slug = %slug, case_id = %case_id, error = %e, "bulk-reset: failed");
+            skipped += 1;
+            continue;
+        }
+        ok += 1;
+    }
+    info!(slug = %slug, ok, skipped, "bulk-reset completed");
+}