Add sample_random_scans script and first-page list-scans option

- scripts/sample_random_scans.sh: pick a random scan per machine (default: first list page) and download mosaic and/or tiles
- --list-scans-first-page-only: one HTTP request for scan list (up to 320 IDs)
- scripts/machines.example.txt; .gitignore local machines.txt (copy from example)
- README: document usage

spruce/session.py

@@ -77,16 +77,28 @@ class MachineSession:
     # Scan list (paginated)
     # ------------------------------------------------------------------
 
-    def get_all_scans(self) -> list[dict[str, Any]]:
+    def get_all_scans(
+        self, first_page_only: bool = False
+    ) -> list[dict[str, Any]]:
         """
-        Fetch the complete scan list across all pages.
+        Fetch the scan list from the RootView table.
 
-        Uses a large FilterCount (320) to minimise round-trips.
-        Falls back to repeated pages if the list is longer.
+        By default, walks all pages. With first_page_only=True, only the first
+        request is made (FilterCount 320) — enough for a random pick without
+        paginating a large history.
         """
+        page_size = 320
+        if first_page_only:
+            all_scans = self._fetch_scan_page(0, page_size)
+            log.info(
+                "[%s] First page only: %d scan(s) (not paginating).",
+                self.machine["label"],
+                len(all_scans),
+            )
+            return all_scans
+
         all_scans: list[dict[str, Any]] = []
         start = 0
-        page_size = 320
 
         while True:
             page_scans = self._fetch_scan_page(start, page_size)