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- Minor cleanups: update legacy report text, improve outdated function docstring

app/services/recommendation_service.py

@@ -56,38 +56,6 @@ def _clamp(value: float, low: float, high: float) -> float:
     return max(low, min(high, value))
 
 
-def _gate_eligible_levels(
-    sr_levels: list[Any],
-    *,
-    confirmed_rounds_only: bool = False,
-    exclude_standalone_rounds: bool = False,
-    min_round_rejections: int = 2,
-) -> list[Any]:
-    """Return structures allowed to influence entry qualification.
-
-    Round numbers remain useful visual landmarks, but an untouched standalone
-    round number is not observed market structure. Research variants can require
-    either confluence with a pivot/volume source or distinct rejection clusters
-    before such a level is allowed to manufacture a gate target.
-    """
-    if not confirmed_rounds_only and not exclude_standalone_rounds:
-        return list(sr_levels)
-
-    eligible: list[Any] = []
-    for level in sr_levels:
-        sources = set(getattr(level, "sources", None) or [
-            getattr(level, "detection_method", "unknown")
-        ])
-        is_round_only = sources == {"round_number"}
-        rejections = int(getattr(level, "rejection_count", 0) or 0)
-        if exclude_standalone_rounds and is_round_only:
-            continue
-        if confirmed_rounds_only and is_round_only and rejections < min_round_rejections:
-            continue
-        eligible.append(level)
-    return eligible
-
-
 def _zone_representative_levels(
     sr_levels: list[SRLevel],
     entry_price: float,