feat: AI cost tracking, pre-launch balance check, low balance alerts

- store/ai_charge.go: local AI cost tracking per call (SQLite)
- wallet/usdc.go: shared USDC balance query (Base chain RPC)
- Pre-launch: estimate daily cost + runway days
- Low balance: warn <$1, error at $0 (every 10 cycles)
- API: GET /api/ai-costs for cost history
- Frontend: model cards show price per call
- Frontend: wallet create + QR deposit + balance display

api/handler_ai_cost.go

@@ -0,0 +1,43 @@
+package api
+
+import (
+	"net/http"
+
+	"github.com/gin-gonic/gin"
+)
+
+// handleGetAICosts returns AI charges for a specific trader
+func (s *Server) handleGetAICosts(c *gin.Context) {
+	traderID := c.Query("trader_id")
+	period := c.DefaultQuery("period", "today")
+
+	if traderID == "" {
+		c.JSON(http.StatusBadRequest, gin.H{"error": "trader_id is required"})
+		return
+	}
+
+	charges, total, err := s.store.AICharge().GetCharges(traderID, period)
+	if err != nil {
+		c.JSON(http.StatusInternalServerError, gin.H{"error": err.Error()})
+		return
+	}
+
+	c.JSON(http.StatusOK, gin.H{
+		"charges": charges,
+		"total":   total,
+		"count":   len(charges),
+	})
+}
+
+// handleGetAICostsSummary returns AI cost summary across all traders
+func (s *Server) handleGetAICostsSummary(c *gin.Context) {
+	period := c.DefaultQuery("period", "today")
+
+	total, count, byModel := s.store.AICharge().GetSummary(period)
+
+	c.JSON(http.StatusOK, gin.H{
+		"total":    total,
+		"count":    count,
+		"by_model": byModel,
+	})
+}