fix(api): use scalar field for rule ownership check

Replace relation filter with scalar emailAccountId field in findUnique query for better security and performance.

- More efficient: avoids unnecessary join operation
- More explicit: directly filters on foreign key column
- Correct Prisma usage: findUnique should use scalar fields, not relation filters

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

apps/web/app/api/user/rules/[id]/route.ts

@@ -15,7 +15,7 @@ async function getRule({
   emailAccountId: string;
 }) {
   const rule = await prisma.rule.findUnique({
-    where: { id: ruleId, emailAccount: { id: emailAccountId } },
+    where: { id: ruleId, emailAccountId },
     include: {
       actions: true,
     },