feat(phase6): per-feature signal attribution in log-odds space

Adds feat_fg_lo / feat_mom_lo / feat_news_lo / feat_mfld_lo / feat_btc_dom_lo
to every trade, all normalized to log-odds contribution for direct comparability.

- fg / mom / btc_dom: raw probability-delta × 2 → log-odds
- news / mfld: already log-odds (LOGODDS_WEIGHT already applied), no scaling
- btc_dom tracked separately in bayesian.py instead of bundled in total_adj
- reasoning string updated to fg_lo= / mom_lo= notation for self-documentation

Schema: 5 new DOUBLE PRECISION columns + 2 partial indexes
Stack: TradingSignal → Order → Trade → save_trade all carry feat fields
Startup: backfill_feature_columns() recovers fg/mom/news/mfld from old
  reasoning strings (×2 applied to fg/mom); btc_dom_lo stays NULL for legacy
API: /api/metrics/features — triggered/material split per feature with
  two-level thresholds (0.05 for fg/mom/btc_dom, 0.10 for news/mfld)
API: /api/trades/legacy — exposes pre-Phase-1 trades (edge_net IS NULL)
API: _enrich_trade backward-compat: reads DB columns first, falls back to
  reasoning regex with unit conversion for pre-Phase-6 trades

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

bot/executor/paper.py

@@ -49,6 +49,12 @@ class Trade:
     commission: float = 0.0      # = POLYMARKET_FEE * size_usdc
     # ── Phase 2: market family ────────────────────────────────────────────────
     family_key: str = ""
+    # ── Phase 6: per-feature log-odds contributions ───────────────────────────
+    feat_fg_lo:      float = 0.0
+    feat_mom_lo:     float = 0.0
+    feat_news_lo:    float = 0.0
+    feat_mfld_lo:    float = 0.0
+    feat_btc_dom_lo: float = 0.0
 
     def __str__(self) -> str:
         return (
@@ -148,6 +154,12 @@ class PaperExecutor:
             commission=commission,
             # Phase 2 family
             family_key=order.family_key,
+            # Phase 6 feature log-odds
+            feat_fg_lo=order.feat_fg_lo,
+            feat_mom_lo=order.feat_mom_lo,
+            feat_news_lo=order.feat_news_lo,
+            feat_mfld_lo=order.feat_mfld_lo,
+            feat_btc_dom_lo=order.feat_btc_dom_lo,
         )
 
         # Update paper portfolio