Add real-time session updates via Server-Sent Events

Implements push-based real-time updates for the Sessions page. New sessions now appear instantly without manual reload or polling.

Changes:
- Add SSE event emitter for session creation notifications
- Create SSE stream endpoint at /api/websites/[id]/sessions/stream
- Emit session events in tracking endpoint when sessions are created
- Add useSessionStream hook to connect to SSE and invalidate queries
- Fix LoadingPanel to prevent flicker during background refetches

src/app/api/send/route.ts

@@ -12,6 +12,7 @@ import { parseRequest } from '@/lib/request';
 import { badRequest, forbidden, json, serverError } from '@/lib/response';
 import { anyObjectParam, urlOrPathParam } from '@/lib/schema';
 import { safeDecodeURI, safeDecodeURIComponent } from '@/lib/url';
+import { emitSessionCreated } from '@/lib/session-events';
 import { createSession, saveEvent, saveSessionData } from '@/queries/sql';
 
 interface Cache {
@@ -151,6 +152,7 @@ export async function POST(request: Request) {
         distinctId: id,
         createdAt,
       });
+      emitSessionCreated(sourceId, sessionId);
     }
 
     // Visit info