imaging dashboard: add Clear all button + endpoint

New /imaging/delete_all endpoint wipes every per-bay JSON in IMAGING_DIR
via imaging_status.delete_all_sessions(). Template adds "Clear all"
outline-danger button next to the count badge, gated on sessions list
non-empty, with confirm() prompt naming the count.

Deployed via scp + systemctl restart pxe-webapp on 172.16.9.1.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

webapp/services/imaging_status.py

@@ -169,3 +169,18 @@ def delete_session(serial: str) -> bool:
         return True
     except OSError:
         return False
+
+
+def delete_all_sessions() -> int:
+    """Wipe every per-bay JSON in IMAGING_DIR. Returns count removed."""
+    _ensure_dir()
+    removed = 0
+    for fn in os.listdir(config.IMAGING_DIR):
+        if not fn.endswith(".json"):
+            continue
+        try:
+            os.unlink(os.path.join(config.IMAGING_DIR, fn))
+            removed += 1
+        except OSError:
+            pass
+    return removed