Update mosaic URL formatting to zero-pad scan IDs to 6 digits

spruce/session.py

@@ -216,8 +216,10 @@ class MachineSession:
     # ------------------------------------------------------------------
 
     def mosaic_url(self, scan_id: int) -> str:
+        # The server stores scan directories zero-padded to 6 digits (e.g. 010700/).
+        # Scans with IDs >= 100000 are unaffected since they are already 6 digits.
         return urljoin(
-            self.image_base_url, f"RootView_Database/{scan_id}/mosaic.jpg"
+            self.image_base_url, f"RootView_Database/{scan_id:06d}/mosaic.jpg"
         )
 
     # ------------------------------------------------------------------
@@ -262,7 +264,7 @@ class MachineSession:
                 ):
                     sc = exc.response.status_code
                 if attempt < retries:
-                    log.debug(
+                    log.warning(
                         "Attempt %d/%d failed %s: %s — retrying in %.0fs",
                         attempt,
                         retries,