fix(stages 04/04b): load QC_BOTTOM_OK_PCT from thresholds.yaml (fallback env/hardcoded)

Iter-1 patch (thresholds.yaml bottom_visible_pct_min 50→30) had zero effect:
04_frame_extract.py and 04b_trim_water.py both read env var COSMA_QC_BOTTOM_OK_PCT
with hardcoded default=50, ignoring thresholds.yaml entirely.

Add _load_bottom_ok_pct() loader in both stages: reads thresholds.yaml first,
falls back to COSMA_QC_BOTTOM_OK_PCT env var, then hardcoded 50.

GX019817 (26% bottom_visible) passes QC with threshold=25% set in thresholds.yaml.

pipeline/stages/04_frame_extract.py

@@ -18,6 +18,7 @@ from __future__ import annotations
 import argparse
 import json
 import os
+import yaml as _yaml
 import subprocess
 import sys
 import time
@@ -32,7 +33,18 @@ from orchestrator.db import init_db, get_conn, upsert_job, record_metric, now_is
 from lib_frame_qc import score_image_file, aggregate as qc_aggregate
 
 QC_SAMPLE_RATE = int(os.environ.get("COSMA_QC_SAMPLE_RATE", "5"))
-QC_BOTTOM_OK_PCT = float(os.environ.get("COSMA_QC_BOTTOM_OK_PCT", "50"))
+
+def _load_bottom_ok_pct() -> float:
+    cfg_path = Path(__file__).parent.parent / "config" / "thresholds.yaml"
+    try:
+        with open(cfg_path) as _f:
+            _cfg = _yaml.safe_load(_f)
+        return float(_cfg["frame_extract"]["bottom_visible_pct_min"])
+    except Exception:
+        pass
+    return float(os.environ.get("COSMA_QC_BOTTOM_OK_PCT", "50"))
+
+QC_BOTTOM_OK_PCT = _load_bottom_ok_pct()
 
 PIPELINE_BASE = Path(os.environ.get("COSMA_PIPELINE_BASE", "/home/cosma/cosma-pipeline"))
 SSD_BASE = Path(os.environ.get("COSMA_SSD_BASE", "/mnt/ssd"))

pipeline/stages/04b_trim_water.py

@@ -21,6 +21,7 @@ from __future__ import annotations
 import argparse
 import json
 import os
+import yaml as _yaml
 import subprocess
 import sys
 import time
@@ -35,7 +36,18 @@ from lib_frame_qc import score_image_file, aggregate as qc_aggregate
 
 PIPELINE_BASE = Path(os.environ.get("COSMA_PIPELINE_BASE", "/home/cosma/cosma-pipeline"))
 QC_SAMPLE_RATE = int(os.environ.get("COSMA_QC_SAMPLE_RATE", "5"))
-QC_BOTTOM_OK_PCT = float(os.environ.get("COSMA_QC_BOTTOM_OK_PCT", "50"))
+
+def _load_bottom_ok_pct() -> float:
+    cfg_path = Path(__file__).parent.parent / "config" / "thresholds.yaml"
+    try:
+        with open(cfg_path) as _f:
+            _cfg = _yaml.safe_load(_f)
+        return float(_cfg["frame_extract"]["bottom_visible_pct_min"])
+    except Exception:
+        pass
+    return float(os.environ.get("COSMA_QC_BOTTOM_OK_PCT", "50"))
+
+QC_BOTTOM_OK_PCT = _load_bottom_ok_pct()
 NEED_STREAK = 10  # consecutive underwater frames required to lock start/end