feat: dynamic Frigate score ceiling; consolidate quality replacement branches

FRIGATE_SCORE_CEILING now defaults to dynamic mode (unset): below-cap
candidates are skipped if their pre-upload Frigate score exceeds the
most-redundant tracked file's score. This catches conditions already
covered by manually-added Frigate images that winnow cannot track —
the embedding-based diversity selection has no visibility into those.
Set FRIGATE_SCORE_CEILING=0 to disable; a positive value (e.g. 0.85)
still acts as a fixed hard ceiling. First-run safety is unchanged
(pre_run_count==0 prevents recognize_face from being called).

The two quality replacement branches (Frigate-score and blur-score)
shared identical structure and are merged into a single code path
parameterised by score source and comparison direction.

Also raises MIN_FACE_COUNT default from 0 to 3 and updates the
config test to match.

tests/test_config.py

@@ -18,7 +18,7 @@ def test_config_loads_defaults(monkeypatch):
     assert cfg.OUTPUT_DIR == "./frigate_train"
     assert cfg.YEARS_FILTER == 10
     assert cfg.MIN_FACE_WIDTH == 90
-    assert cfg.MIN_FACE_COUNT == 0
+    assert cfg.MIN_FACE_COUNT == 3
     assert cfg.BLUR_THRESHOLD == 120.0
     assert cfg.MIN_CONFIDENCE == 0.7
     assert cfg.MAX_AUTO_IMAGES == 80