feat: adopt Phase 3 gate and paper-trade exit policy

Production strategy change based on the July 2026 backtest: paper trades now default to a 30-trading-day hold with the initial stop (classic momentum hold-and-rerank), while target and trailing exits remain available in Admin. The exit policy API/UI now carries hold_days and close_reason can be 'time'.

The activation confidence floor default is now 0/off because the gate ablation showed it added no per-trade edge while filtering out usable setups. Migration 015 clears stored activation_min_confidence and paper_exit_mode so the new defaults take effect; this intentionally resets Track Record comparability from this deploy.

Verification: 451 backend tests pass, ruff check app/ clean, frontend npm run build clean.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

app/schemas/paper_trade.py

@@ -22,8 +22,9 @@ class PaperTradeClose(BaseModel):
 
 class ExitPolicyUpdate(BaseModel):
     """Auto-exit policy for open paper trades."""
-    mode: str | None = Field(default=None, pattern=r"^(trailing|target)$")
+    mode: str | None = Field(default=None, pattern=r"^(time|trailing|target)$")
     trailing_pct: float | None = Field(default=None, ge=0.5, le=90)
+    hold_days: int | None = Field(default=None, ge=2, le=250)
 
 
 class PaperTradeResponse(BaseModel):