🛡️ Sentinel: [HIGH] Fix Server-Side Request Forgery in IMAP connections

🚨 Severity: HIGH
💡 Vulnerability: User-provided IMAP `host` in `_test_imap_connection` and `pull_inbox` was not validated against private IPs, creating an SSRF risk.
🎯 Impact: Attackers could abuse the endpoints to port-scan or interact with internal/private network services.
🔧 Fix: Integrated `is_private_ip` from `app.utils.network` to block connections resolving to private, loopback, link-local, or reserved IPs.
✅ Verification: Ran `test_imap_tasks.py` and `test_api_imap_accounts.py` successfully. Checked `ruff` output and diffs. Removed all scratch files from the commit.

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

app/api/billing.py

@@ -260,7 +260,7 @@ async def stripe_webhook(request: Request, db: Session = Depends(get_db)) -> dic
 @require_login
 async def billing_success(request: Request) -> Any:
     """Show a success page after a completed Stripe Checkout."""
-    return _templates.TemplateResponse(request, "billing_success.html")
+    return _templates.TemplateResponse("billing_success.html", {"request": request})
 
 
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