feat: automate exact dedupe and improve web feedback

am_deduper/locking.py

@@ -0,0 +1,18 @@
+from __future__ import annotations
+
+from contextlib import contextmanager
+from pathlib import Path
+import fcntl
+
+
+@contextmanager
+def exclusive(db_path: Path):
+    """Cross-process lock for destructive dedupe operations."""
+    lock_path = Path(str(db_path) + ".lock")
+    lock_path.parent.mkdir(parents=True, exist_ok=True)
+    with lock_path.open("w") as handle:
+        fcntl.flock(handle.fileno(), fcntl.LOCK_EX)
+        try:
+            yield
+        finally:
+            fcntl.flock(handle.fileno(), fcntl.LOCK_UN)