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/pyproject.toml

@@ -22,41 +22,37 @@ dependencies = [
     "pydantic-settings>=2.2.1",
     "python-multipart>=0.0.19",
     "fastapi-utils==0.8.0",
-
     # Database
     "sqlalchemy>=2.0.29",
     "alembic>=1.14.1",
     "psycopg2-binary>=2.9.9",
     "asyncpg>=0.29.0",
     "aiosqlite==0.21.0",
-
     # Environment configuration
     "python-dotenv>=1.0.1",
-
     # API utilities
     "email-validator>=2.1.0.post1",
     "ujson>=5.9.0",
-
     # CORS and security
     "starlette>=0.40.0",
     "starlette-csrf>=1.4.5",
     "slowapi>=0.1.9",
-
     # Utilities
     "httpx>=0.27.0",
     "tenacity>=8.2.3",
     "pytz>=2024.1",
     "pillow>=10.3.0",
     "apscheduler==3.11.0",
-
     # Security and authentication (pinned for reproducibility)
     "python-jose==3.4.0",
     "passlib==1.7.4",
     "bcrypt==4.2.1",
     "cryptography==44.0.1",
-
     # OAuth authentication
     "authlib>=1.3.0",
+    # Celery for background task processing (Syndarix agent jobs)
+    "celery[redis]>=5.4.0",
+    "sse-starlette>=3.1.1",
 ]
 
 # Development dependencies