feat: Scaffold Novi-Mailman Bridge integration

- app/methods/e_novi_mailman_methods.py: full sync engine, per-member
  sync helper, webhook handler, and Mailman 3 REST subscribe/unsubscribe
- app/routers/api_v3_actions_e_novi_mailman.py: test_connection, list
  inspection, full sync trigger, and Novi webhook receiver endpoints
- registry.py: registered at /v3/action/e_novi_mailman
- TODO: marked as scaffolded, pending Novi field verification + data_store setup

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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, api_v3_actions_event_file, api_v3_actions_e_zoom, lookup, lookup_v3,
+    flask_cfg, hosted_file, api_v3_actions_hosted_file, api_v3_actions_event_file, api_v3_actions_e_zoom, api_v3_actions_e_novi_mailman, lookup, lookup_v3,
     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
@@ -51,6 +51,7 @@ def setup_routers(app: FastAPI):
     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(api_v3_actions_e_zoom.router, prefix='/v3/action/e_zoom', tags=['Zoom Events (V3 Actions)'])
+    app.include_router(api_v3_actions_e_novi_mailman.router, prefix='/v3/action/e_novi_mailman', tags=['Novi-Mailman Bridge (V3 Actions)'])
     app.include_router(lookup.router, prefix='/lu', tags=['Lookup'])
     app.include_router(lookup_v3.router, prefix='/v3/lookup', tags=['Lookup V3'])