feat(kevin): mention-driven backtest mini-engine

Walks mentions chronologically, T+1 entry, time-based exit per
KevinStrategy. Reuses backtester/metrics::compute_metrics for headline
numbers. KevinPriceLoader fronts market_data + Alpaca.

shared/strategies/kevin.py

@@ -57,8 +57,10 @@ class KevinStrategy:
         is_tradable: bool,
     ) -> KevinDecision:
         symbol = mention.symbol
-        action = mention.action
-        horizon = mention.time_horizon
+        # Normalize the action/horizon to their str value so the strategy works
+        # with both SQLAlchemy enum instances and lightweight stubs (backtest).
+        action_value = getattr(mention.action, "value", mention.action)
+        horizon_value = getattr(mention.time_horizon, "value", mention.time_horizon)
 
         # 1. Common no-trade gates
         if not is_tradable:
@@ -76,15 +78,15 @@ class KevinStrategy:
             )
 
         # 2. Action-specific gates
-        if action in (TickerAction.HOLD, TickerAction.WATCH):
+        if action_value in (TickerAction.HOLD.value, TickerAction.WATCH.value):
             return KevinDecision(
                 decision=KevinDecisionType.NO_OP,
                 symbol=symbol,
-                rationale=f"action={action.value} is UI-only, never trades",
+                rationale=f"action={action_value} is UI-only, never trades",
             )
 
         # 3. SELL — close long if held, else no-op
-        if action == TickerAction.SELL:
+        if action_value == TickerAction.SELL.value:
             if account.is_held(symbol):
                 return KevinDecision(
                     decision=KevinDecisionType.CLOSE_LONG,
@@ -99,7 +101,7 @@ class KevinStrategy:
             )
 
         # 4. AVOID — close long if held + bridge will add blocklist (side effect)
-        if action == TickerAction.AVOID:
+        if action_value == TickerAction.AVOID.value:
             if account.is_held(symbol) and self.config.avoid_closes_longs:
                 return KevinDecision(
                     decision=KevinDecisionType.CLOSE_LONG,
@@ -120,7 +122,7 @@ class KevinStrategy:
             )
 
         # 5. BUY path — full filter stack
-        assert action == TickerAction.BUY
+        assert action_value == TickerAction.BUY.value
 
         if effective_conviction < self.config.min_conviction:
             return KevinDecision(
@@ -132,7 +134,7 @@ class KevinStrategy:
                 ),
             )
 
-        if horizon == TimeHorizon.INTRADAY:
+        if horizon_value == TimeHorizon.INTRADAY.value:
             return KevinDecision(
                 decision=KevinDecisionType.NO_OP,
                 symbol=symbol,
@@ -197,7 +199,7 @@ class KevinStrategy:
         target_dollars = target_dollars.quantize(Decimal("0.01"))
 
         holding_days = self.config.hold_days_by_horizon.get(
-            horizon.value, self.config.hold_days_by_horizon["unspecified"]
+            horizon_value, self.config.hold_days_by_horizon["unspecified"]
         )
 
         return KevinDecision(