fix: get_export_folders respects scan_export filter

Ghost folders (scan-export-only) no longer appear in training dropdowns.
Also filters out 0-clip folders from get_training_stats.

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

tests/test_db.py

@@ -0,0 +1,25 @@
+import os
+import tempfile
+
+from core.db import ProcessedDB
+
+
+def test_export_folders_excludes_scan_exports():
+    """Scan-export-only folders should not appear when include_scan_exports=False."""
+    with tempfile.NamedTemporaryFile(suffix=".db", delete=False) as f:
+        path = f.name
+    try:
+        db = ProcessedDB(path)
+        # Manual export
+        db.add("a.mp4", 10.0, "/out/mp4_Intense/g1/clip.mp4", profile="test")
+        # Scan export to different folder
+        db.add("a.mp4", 20.0, "/out/mp4_ScanOnly/g1/clip.mp4", profile="test",
+               scan_export=True)
+        folders = db.get_export_folders("test")
+        assert "mp4_Intense" in folders
+        assert "mp4_ScanOnly" not in folders, "scan-only folder should be excluded"
+        # With include_scan_exports=True, both should appear
+        folders_all = db.get_export_folders("test", include_scan_exports=True)
+        assert "mp4_ScanOnly" in folders_all
+    finally:
+        os.unlink(path)