docs: document WhatsApp vs Signal detection logic

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

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

Author: MarkusNeusinger

docs/architecture/plausible.md

@@ -164,6 +164,16 @@ Bot requests page → nginx detects bot → SEO proxy serves HTML with og:image
 
 **Fallback**: unknown
 
+#### WhatsApp vs Signal Detection
+
+Signal deliberately uses a WhatsApp User-Agent to bypass rate limits on sites like Twitter ([Issue #10060](https://github.com/signalapp/Signal-Android/issues/10060)). We distinguish them by version format:
+
+| Platform | User-Agent Example | Detection |
+|----------|-------------------|-----------|
+| WhatsApp (iOS) | `WhatsApp/2.23.18.78 i` | 3+ part version → whatsapp |
+| WhatsApp (Android) | `WhatsApp/2.21.22.23 A` | 3+ part version → whatsapp |
+| Signal | `WhatsApp` or `WhatsApp/2` | No full version → signal |
+
 ### API Endpoints
 
 | Endpoint | Description | Tracking |