Cleanup retired S/R research scaffolding

- Remove unused _gate_eligible_levels filtering logic and its tests (research-only)
- Add prominent RESEARCH/DIAGNOSTIC markers and docs to clear-air/ATR fallback helpers
- Document production vs research BACKTEST_* environment variables in backtest_service
- Minor cleanups: update legacy report text, improve outdated function docstring

tests/unit/test_recommendation_service.py

@@ -5,7 +5,6 @@ from dataclasses import dataclass
 from app.services.recommendation_service import (
     _build_reasoning,
     _choose_recommended_action,
-    _gate_eligible_levels,
     _prune_floor_pinned_targets,
     _select_primary_target,
     direction_analyzer,
@@ -354,26 +353,3 @@ def test_zone_representative_levels_soft_strength_avoids_resaturation():
     assert reps[0].strength == 65
     assert set(reps[0].sources) == {"pivot_point", "round_number"}
     assert reps[0].rejection_count == 3
-
-
-def test_gate_requires_confirmation_for_standalone_round_number():
-    from types import SimpleNamespace
-
-    untouched = SimpleNamespace(
-        detection_method="round_number", sources=["round_number"],
-        rejection_count=1,
-    )
-    confirmed = SimpleNamespace(
-        detection_method="round_number", sources=["round_number"],
-        rejection_count=2,
-    )
-    confluent = SimpleNamespace(
-        detection_method="merged", sources=["round_number", "pivot_point"],
-        rejection_count=0,
-    )
-    assert _gate_eligible_levels(
-        [untouched, confirmed, confluent], confirmed_rounds_only=True
-    ) == [confirmed, confluent]
-    assert _gate_eligible_levels(
-        [untouched, confirmed, confluent], exclude_standalone_rounds=True
-    ) == [confluent]