feat(event-file): implement atomic V3 upload action for event files

- Creates api_v3_actions_event_file.py with a specialized /upload endpoint.
- Handles physical storage (hosted_file), generic linking, and event association (event_file) in one request.
- Implements intelligent ID resolution to prevent duplicate event associations for the same physical file.
- Updates documentation in GUIDE__V3_FRONTEND_API.md.

app/routers/registry.py

@@ -7,7 +7,7 @@ from app.routers import (
     event_device, event_exhibit, event_exhibit_tracking, event_file, event_importing,
     event_location, event_person,
     event_presentation, event_presenter, event_session,
-    flask_cfg, hosted_file, api_v3_actions_hosted_file, lookup,
+    flask_cfg, hosted_file, api_v3_actions_hosted_file, api_v3_actions_event_file, lookup,
     organization, page, person,
     person_user, qr, site, site_domain, user,
     util_email, websockets, websockets_redis, websockets_v3, e_confex, e_cvent, e_impexium, e_stripe
@@ -48,6 +48,7 @@ def setup_routers(app: FastAPI):
 
     app.include_router(hosted_file.router, prefix='/hosted_file', tags=['Hosted File'])
     app.include_router(api_v3_actions_hosted_file.router, prefix='/v3/action/hosted_file', tags=['Hosted File (V3 Actions)'])
+    app.include_router(api_v3_actions_event_file.router, prefix='/v3/action/event_file', tags=['Event File (V3 Actions)'])
     app.include_router(lookup.router, prefix='/lu', tags=['Lookup'])
 
     # app.include_router(organization.router, prefix='/organization', tags=['Organization'], dependencies=[Depends(DeprecationParams)])