feat(#357): W4c — Custom-Fields-Routen aus Core in ContactsPlugin migriert

Kritikpunkt 14: app/routes/custom_fields.py war 100% Contact-spezifisch
(importiert Contact, nutzt contacts:read/write, Route /{contact_id}/custom-fields)
aber lag als scheinbar generischer Core-Service.

Fix: Die komplette Logik (2 Endpoints GET/PATCH /{contact_id}/custom-fields,
_collect_custom_field_definitions, _merge_definitions_with_values) wandert in
app/plugins/builtins/contacts/routes.py (gleicher Router-Prefix /api/v1/contacts).
app/routes/custom_fields.py geloescht, main.py bereinigt.

generischer custom_field_definitions.py-Endpoint bleibt im Core (echtes Core-
Entity). Frontend-Endpoint-Shapes unveraendert.

Funktionserhalt bewiesen: 11/11 tests/test_custom_fields.py passed.

fixes #357 (W4c-Teil)

app/main.py

@@ -45,7 +45,6 @@ from app.routes import (  # noqa: E402
     compliance,
     currencies,
     custom_field_definitions,
-    custom_fields,
     dashboard,
     delegations,
     entity_history,
@@ -584,7 +583,6 @@ def create_app() -> FastAPI:
     app.include_router(compliance.router)
     app.include_router(owner_transfer.router)
     app.include_router(custom_field_definitions.router)
-    app.include_router(custom_fields.router)
     app.include_router(saved_filters.router)
     app.include_router(saved_views.router)
     app.include_router(webhooks.router)