fix: wrap contour detection and pipeline in catch_unwind

imageproc::find_contours panics on some edge images (called
Option::unwrap() on None). Wrap with catch_unwind and provide
a safe fallback (use full image bounds).

Also add catch_unwind around the entire pipeline so no panic
can crash the worker thread.

Co-Authored-By: Kilo <kilo@kilo.ai>

backend/workers/src/scanner/pipeline.rs

@@ -26,10 +26,32 @@ pub struct ScanResult {
 }
 
 /// Run the full scanner pipeline with all stages.
+/// Wrapped in catch_unwind to prevent imageproc panics from killing the worker.
 pub async fn process(
     job: &Job,
     config: &WorkerConfig,
     progress: &ProgressReporter,
+) -> Result<ScanResult, Box<dyn std::error::Error + Send + Sync>> {
+    let result = std::panic::catch_unwind(std::panic::AssertUnwindSafe(|| {
+        process_inner(job, config, progress)
+    }));
+
+    match result {
+        Ok(fut) => fut.await,
+        Err(panic) => {
+            let msg = panic
+                .downcast_ref::<&str>()
+                .unwrap_or(&"Unknown panic in scanner pipeline");
+            Err(format!("Pipeline panicked: {}", msg).into())
+        }
+    }
+}
+
+/// Inner pipeline implementation (runs inside catch_unwind).
+async fn process_inner(
+    job: &Job,
+    config: &WorkerConfig,
+    progress: &ProgressReporter,
 ) -> Result<ScanResult, Box<dyn std::error::Error + Send + Sync>> {
     let start = Instant::now();
     let input_path = Path::new(&job.file_path);