feat: add Claw402 (claw402.ai) x402 USDC payment provider

Add Claw402Client for claw402.ai's x402 micropayment gateway (Base USDC).
Supports 15+ AI models (GPT-5.4, Claude Opus, DeepSeek, Qwen, Grok, etc.)
with per-model endpoint routing.

- mcp/claw402.go: new client with model→endpoint mapping, x402 v2 payment flow
- mcp/blockrun_base.go: extract shared signX402Payment() for reuse
- Register "claw402" provider in all 6 consumer switch statements:
  api/server.go, api/strategy.go, trader/auto_trader.go,
  telegram/bot.go, debate/engine.go, backtest/ai_client.go

api/server.go

@@ -3450,6 +3450,7 @@ func (s *Server) handleGetSupportedModels(c *gin.Context) {
 		{"id": "minimax", "name": "MiniMax", "provider": "minimax", "defaultModel": "MiniMax-M2.5"},
 		{"id": "blockrun-base", "name": "BlockRun (Base Wallet)", "provider": "blockrun-base", "defaultModel": "auto"},
 		{"id": "blockrun-sol", "name": "BlockRun (Solana Wallet)", "provider": "blockrun-sol", "defaultModel": "auto"},
+		{"id": "claw402", "name": "Claw402 (Base USDC)", "provider": "claw402", "defaultModel": "deepseek"},
 	}
 
 	c.JSON(http.StatusOK, supportedModels)