feat(backtest): wire feature-aware models into the backtesting fold loop (#244)

[w7-mgfcode]

Implements PRPs/PRP-MLZOO-B.2-feature-aware-backtesting.md. Closes #244.

Feature-aware models (regression, lightgbm — any model with
requires_features=True) are now evaluated by POST /backtesting/run and
backtest jobs, with per-fold leakage-safe X_train / X_future matrices.
This is the follow-up deferred by PRP-30 DECISIONS LOCKED #6.

What changed

• Shared row builders — app/shared/feature_frames/rows.py:
  build_historical_feature_rows (promoted verbatim from
  ForecastingService._assemble_regression_rows) + build_future_feature_rows,
  the leakage-critical per-fold test-window assembler. _assemble_regression_rows
  becomes a delegating shim — its load-bearing leakage spec stays green, unedited.
• BacktestingService — resolves exogenous data once (async ExogenousFrame),
  branches the fold loop on requires_features, slices X_train from one
  historical matrix, rebuilds X_future per fold (never sliced — that would leak
  an adjacent test-day target). min_train_size >= 30 enforced loud.
• ModelBacktestResult — additive feature_aware / exogenous_policy fields;
  _shape_backtest_result keys unchanged (frontend contract byte-stable).
• JobService._execute_backtest — accepts regression + lightgbm.
• Interim loud-fail test repurposed (PRP-29 chore(main): release 0.1.1 #7 / PRP-30 fix: remove CRLF line endings from pyproject.toml #6 superseded); new shared
  X_future leakage spec including a gap>0 fold.

Scope

No frontend/, scenarios/, or alembic/ change; no migration.

Validation

• ruff + mypy --strict + pyright --strict — clean
• unit — 1341 passed (incl. test_regression_features_leakage.py unedited)
• integration — 20 passed (regression backtest + baselines vs real Postgres)
• dogfood — regression backtest feature_aware:true/exogenous_policy:"observed";
  min_train<30 → 400; backtest job completes; naive baseline unaffected

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