Add SignalR security gates to .NET API review

[99INFLUENCERS]

Summary

• Adds a SignalR-specific review section to the .NET API security supplement.
• Covers hub inventory, browser-origin policy, hub/method authorization, group membership checks, query-string access-token scoping, log redaction, detailed errors, and buffer limits.
• Adds grep patterns for SignalR hub endpoints/classes, group/broadcast operations, query-token handling, and risky production settings.

Related review

Closes #1187

Bounty request

Improver - Moderate ($100) if accepted. Preferred payment method: PayPal; details can be provided privately after maintainer acceptance.

Validation

• git diff --check
• Markdown fence balance check for skills/appsec/api-security/csharp-dotnet.md
• Content assertions for the new SignalR headings and grep section
• Official Microsoft Learn reference URLs returned HTTP 200