fix: AI 预测失败时显示明确错误原因

- Real 模式下无 API Key 时提示「未配置 AI API Key」
- SSE 错误端点返回真实错误消息而非通用提示

backend/src/routes/ai.ts

@@ -17,8 +17,8 @@ async function predictFormula(request: FastifyRequest<{ Body: { ingredients: Arr
   try {
     const result = await aiService.predictMetrics(ingredients)
     reply.raw.write(`data: ${JSON.stringify({ type: 'result', content: result })}\n\n`)
-  } catch {
-    reply.raw.write(`data: ${JSON.stringify({ type: 'error', content: '预测失败' })}\n\n`)
+  } catch (err) {
+    reply.raw.write(`data: ${JSON.stringify({ type: 'error', content: (err as Error).message })}\n\n`)
   }
   reply.raw.end()
 }
@@ -44,8 +44,8 @@ async function exploreFormula(request: FastifyRequest<{ Body: {
     for (const option of parsed) {
       reply.raw.write(`data: ${JSON.stringify({ type: 'option', option })}\n\n`)
     }
-  } catch {
-    reply.raw.write(`data: ${JSON.stringify({ type: 'error', content: '推演失败' })}\n\n`)
+  } catch (err) {
+    reply.raw.write(`data: ${JSON.stringify({ type: 'error', content: (err as Error).message })}\n\n`)
   }
   reply.raw.write(`data: ${JSON.stringify({ type: 'done' })}\n\n`)
   reply.raw.end()

backend/src/services/ai/index.ts

@@ -136,6 +136,10 @@ export class AIService {
       return MOCK_RESPONSES[capability] ?? '{}'
     }
 
+    if (Object.keys(this.providers).length === 0) {
+      throw new Error('未配置 AI API Key，请在设置中配置或切换到 Mock 模式')
+    }
+
     await this.rateLimiter.acquire()
 
     const { provider, model } = this.selectProvider(opts.model)