feat(backend): Add SSE endpoint for project event streaming

- Add /projects/{project_id}/events/stream SSE endpoint
- Add event_bus dependency injection
- Add project access authorization (placeholder)
- Add test event endpoint for development
- Add keepalive comments every 30 seconds
- Add reconnection support via Last-Event-ID header
- Add rate limiting (10/minute per IP)
- Mount events router in API
- Add sse-starlette dependency
- Add 19 comprehensive tests for SSE functionality

Implements #34

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

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

backend/app/api/main.py

@@ -3,6 +3,7 @@ from fastapi import APIRouter
 from app.api.routes import (
     admin,
     auth,
+    events,
     oauth,
     oauth_provider,
     organizations,
@@ -22,3 +23,5 @@ api_router.include_router(admin.router, prefix="/admin", tags=["Admin"])
 api_router.include_router(
     organizations.router, prefix="/organizations", tags=["Organizations"]
 )
+# SSE events router - no prefix, routes define full paths
+api_router.include_router(events.router, tags=["Events"])