feat(ST-351): add UserAvatar component and /api/users/[id]/avatar route

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

app/api/users/[id]/avatar/route.ts

@@ -0,0 +1,28 @@
+import { prisma } from '@/lib/prisma'
+import { requireUser } from '@/lib/auth'
+
+export async function GET(_request: Request, { params }: { params: Promise<{ id: string }> }) {
+  try {
+    await requireUser()
+  } catch {
+    return new Response(null, { status: 401 })
+  }
+
+  const { id } = await params
+
+  const user = await prisma.user.findUnique({
+    where: { id },
+    select: { avatar_data: true },
+  })
+
+  if (!user?.avatar_data) {
+    return new Response(null, { status: 404 })
+  }
+
+  return new Response(user.avatar_data, {
+    headers: {
+      'Content-Type': 'image/webp',
+      'Cache-Control': 'private, max-age=3600',
+    },
+  })
+}