feat: scan result history — keep N versions per (file, model)

Add scan_timestamp column to scan_results. save_scan_results now inserts
with a timestamp and prunes versions beyond max_versions (default 5).
get_scan_results returns only the latest version by default, with optional
scan_timestamp parameter for loading specific versions. New get_scan_versions
method returns available versions for a (file, profile, model) tuple.

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

tests/test_db.py

@@ -1,5 +1,6 @@
 import os
 import tempfile
+import time
 
 from core.db import ProcessedDB
 
@@ -23,3 +24,29 @@ def test_export_folders_excludes_scan_exports():
         assert "mp4_ScanOnly" in folders_all
     finally:
         os.unlink(path)
+
+
+def test_scan_result_history():
+    """save_scan_results should keep multiple versions."""
+    with tempfile.NamedTemporaryFile(suffix=".db", delete=False) as f:
+        path = f.name
+    try:
+        db = ProcessedDB(path)
+        # Save three versions with small delays so timestamps differ
+        db.save_scan_results("v.mp4", "test", "MODEL_A", [(0, 8, 0.9)])
+        time.sleep(1.1)
+        db.save_scan_results("v.mp4", "test", "MODEL_A",
+                             [(0, 8, 0.8), (10, 18, 0.7)])
+        time.sleep(1.1)
+        db.save_scan_results("v.mp4", "test", "MODEL_A", [(5, 13, 0.95)])
+        versions = db.get_scan_versions("v.mp4", "test", "MODEL_A")
+        assert len(versions) == 3
+        # Most recent first
+        assert versions[0]["count"] == 1   # latest: 1 region
+        assert versions[1]["count"] == 2   # middle: 2 regions
+        assert versions[2]["count"] == 1   # oldest: 1 region
+        # get_scan_results returns latest version by default
+        results = db.get_scan_results("v.mp4", "test")
+        assert len(results.get("MODEL_A", [])) == 1
+    finally:
+        os.unlink(path)