Add custom filter for DWS and Tanishq Dubey trademarks

- Create custom_filter.go with DWS/Tanishq Dubey term detection
- Block variations including:
  - DWS, Dubey Web Services, DWS Engineering LLC
  - Tanishq Dubey, tdubey
  - Leet speak variations (dub3y, t4nishq, dw5, etc.)
  - Combined terms (dubeydns, dws-ddns, etc.)
- Update frontend to show 'reserved' message for blocked terms
- Filters are case-insensitive and handle separators

Protects brand identity and personal name from being used
in user subdomains

web/static/js/app.js

@@ -37,6 +37,10 @@ document.addEventListener('DOMContentLoaded', function() {
                 availabilityStatus.textContent = '✗ Contains inappropriate content';
                 availabilityStatus.className = 'status taken';
                 claimBtn.disabled = true;
+            } else if (data.reason === 'reserved') {
+                availabilityStatus.textContent = '✗ This subdomain is reserved';
+                availabilityStatus.className = 'status taken';
+                claimBtn.disabled = true;
             } else {
                 availabilityStatus.textContent = '✗ Already taken';
                 availabilityStatus.className = 'status taken';