docs(developer-guide): add Server-Sent Events guide

Document the SSE infrastructure for backend developers: when to use SSE,
the architecture and ASGI transport, a step-by-step worked example for
adding an endpoint to a feature, the resource.verb event-naming
convention, authentication, the tenant-isolation model, and reconnect/
state-recovery. Register the page in the Developer Guide navigation.

docs/docs.json

@@ -395,7 +395,8 @@
               "developer-guide/lighthouse-architecture",
               "developer-guide/mcp-server",
               "developer-guide/ai-skills",
-              "developer-guide/prowler-studio"
+              "developer-guide/prowler-studio",
+              "developer-guide/server-sent-events"
             ]
           },
           {