[Refactor/Chore] Add persisted EndUser type enum

[laipz8200]

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Description

The end_users.type field is currently written from several backend entry points using raw strings or invoke-source enum values. This makes the persisted values harder to audit and allows call sites to drift as new surfaces create end users.

Introduce a dedicated EndUserType enum for the persisted end_users.type values and update backend end-user creation paths to use it consistently.

Motivation

A dedicated enum makes the database contract explicit, keeps direct EndUser construction consistent across service API, web app, MCP, OpenAPI, and trigger paths, and prevents accidental reuse of unrelated invoke-source enums for persisted end-user type values.

Additional Context

Current production values include browser, service-api, service_api, mcp, and trigger. OpenAPI is also represented by current backend creation paths.