Add user existence check API and link existing users to organizations

[LinceMathew]

Add User Existence Check API and integrate it into the UI.

Overview

This change introduces a new API endpoint to check for existing users by email. The UI is updated to leverage this check, streamlining user creation and management. Default AI connectors are also removed during specific plan downgrades.

Technical Highlights

• internal/api/server.go: A new API endpoint /orgs/:org_id/users/check is added.
• internal/api/users/handlers.go: A CheckUser handler is implemented to query user existence.
• internal/api/users/user_service.go: A CheckUserByEmail method is added to the user service.
• ui/src/api/users.ts: A checkUserByEmail function is added to the user API client.
• ui/src/components/UserManagement/UserForm.tsx: The user form integrates the email check API for dynamic validation.
• storage/license/plan_change_store.go: Logic is added to remove the default AI connector on "free_30k" downgrade.
• storage/payment/subscription_store.go: Default AI connector deletion is added for various free tier downgrade scenarios.
• .air.toml: OpenAPI spec file is excluded from live reloading.