Clean all /dev/shm counters on batch start

Wipe chicken_counter_* directories on batch start so dashboard
starts clean and populates as each camera runs.

src/chicken_counter/batch_runner.py

@@ -2,6 +2,7 @@
 
 from __future__ import annotations
 
+import shutil
 from datetime import date as date_type
 from pathlib import Path
 
@@ -75,6 +76,13 @@ def run_daily_batch(settings: BatchSettings, date: str | None = None, *, verbose
     print(f"[batch] starting daily run for {run_date}")
     print(f"[batch] input folder: {day_dir}")
     print(f"[batch] output folder: {output_dir}")
+
+    # Clean all /dev/shm counters from previous runs
+    shm_dir = Path("/dev/shm")
+    for d in shm_dir.glob("chicken_counter_*"):
+        if d.is_dir():
+            shutil.rmtree(str(d))
+            print(f"[batch] cleaned {d}")
     if no_video:
         print("[batch] --no-video: skipping video output, overlay, and compression")