Add getAuthUser() auth guard to all ML proxy API routes (task 7.3)

Adds authentication check at the top of each handler in:
- /api/predict
- /api/predict/batch
- /api/model/info
- /api/model/load
- /api/patterns/detect
- /api/patterns/available
- /api/training/start
- /api/training/runs

Returns 401 Unauthorized for unauthenticated requests. Proxy/fetch logic unchanged.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/app/api/model/load/route.ts

@@ -1,5 +1,6 @@
 import { NextRequest, NextResponse } from 'next/server';
 import { z } from 'zod';
+import { getAuthUser } from '@/lib/auth';
 
 const INFERENCE_API_URL = process.env.INFERENCE_API_URL || 'http://localhost:8001';
 const INFERENCE_API_TIMEOUT = parseInt(process.env.INFERENCE_API_TIMEOUT || '30000', 10);
@@ -9,6 +10,11 @@ const ModelLoadRequestSchema = z.object({
 });
 
 export async function POST(request: NextRequest) {
+  const user = await getAuthUser();
+  if (!user) {
+    return NextResponse.json({ error: 'Unauthorized' }, { status: 401 });
+  }
+
   const controller = new AbortController();
   const timeoutId = setTimeout(() => controller.abort(), INFERENCE_API_TIMEOUT);