feat(websockets): implement WebSockets V3 with granular Redis Pub/Sub

- Introduced WS_Message_V3 standardized Pydantic model and WS_Manager_V3.
- Implemented /v3/ws/ endpoint with granular Redis routing to solve "noisy neighbor" scaling issues.
- Added presence tracking using Redis Sets for group coordination.
- Comprehensive test suite added (unit and integration) covering models, manager, and routing logic.
- Documentation: Created V3 Frontend WebSocket Guide and Project design spec.
- Updated main Frontend API guide and tests README with new standards.

app/routers/registry.py

@@ -10,7 +10,7 @@ from app.routers import (
     flask_cfg, hosted_file, api_v3_actions_hosted_file, lookup,
     organization, page, person,
     person_user, qr, site, site_domain, user,
-    util_email, websockets, websockets_redis, e_confex, e_cvent, e_impexium, e_stripe
+    util_email, websockets, websockets_redis, websockets_v3, e_confex, e_cvent, e_impexium, e_stripe
 )
 
 def setup_routers(app: FastAPI):
@@ -62,6 +62,7 @@ def setup_routers(app: FastAPI):
     app.include_router(util_email.router, tags=['Utility: Email'])
     app.include_router(websockets.router, tags=['Websockets'])
     app.include_router(websockets_redis.router, tags=['Websockets (Redis)'])
+    app.include_router(websockets_v3.router, prefix='/v3', tags=['Websockets V3'])
 
     app.include_router(e_confex.router, prefix='/e/confex', tags=['External Service: Confex'])
     app.include_router(e_cvent.router, prefix='/e/cvent', tags=['External Service: Cvent'])