fix: timestamp collision, undo stack invalidation, label parsing, filter-aware clear

- Use microsecond-precision timestamps to prevent version merging on
  sub-second scans
- Clear undo stack when switching scan versions (stale row references)
- Parse timestamp labels robustly instead of hard-coded string slicing
- "Clear All" in hard negatives dialog respects active model filter
- Remove time.sleep from tests (no longer needed with microsecond timestamps)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

core/db.py

@@ -504,7 +504,7 @@ class ProcessedDB:
         """
         if not self._enabled:
             return
-        ts = datetime.now().strftime("%Y%m%d_%H%M%S")
+        ts = datetime.now().strftime("%Y%m%d_%H%M%S_%f")
         with self._lock:
             self._con.executemany(
                 "INSERT INTO scan_results"