Pull Request Severity-Graded Code Review

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PR Title: feat(billing): Add Stripe subscription webhook handler
Description / Intent: Handles customer.subscription.deleted and invoice.payment_failed webhooks to revoke user tenant seats.

Git Diff / Code Changes:
```diff
+ async function handleWebhook(req, res) {
+   const event = req.body;
+   if (event.type === 'customer.subscription.deleted') {
+     const sub = event.data.object;
+     await db.query(`UPDATE users SET active = false WHERE stripe_customer_id = '${sub.customer}'`);
+   }
+   res.status(200).send('OK');
+ }
```

Review this PR with high technical rigor. Group every comment by severity level:

### 🔴 Blocker (Must fix before merge)
Issues that cause crashes, data corruption, security vulnerabilities, breaking API changes, or severe performance degradation.

### 🟠 Major (Strongly recommended)
Architecture flaws, missing error handling, lack of essential test coverage, race conditions, or bad database queries.

### 🟡 Minor (Improvements)
Code readability, non-standard naming, small optimizations, or documentation gaps.

### ⚪ Nitpick (Non-blocking preferences)
Stylistic preferences, typo fixes, or minor formatting tweaks.

For each finding:
1. Reference the exact file and line range.
2. Explain **why** it is an issue with concrete failure scenarios.
3. Provide the exact suggested **code diff** to fix it.