Desktop: subscription endpoints hit Rust backend (404) instead of Python backend

[mdmohsin7]

Bug

Desktop app users see "Failed to load plan information" when navigating to the Plan & Usage section in Settings.

Root Cause

The desktop app sends all API calls to the Rust desktop backend (OMI_API_URL, which resolves to desktop-backend-hhibjajaja-uc.a.run.app in production). However, the subscription and payment endpoints only exist on the Python backend (api.omi.me):

• GET /v1/users/me/subscription → 404 on Rust backend
• POST /v1/payments/checkout-session → 404 on Rust backend
• POST /v1/payments/customer-portal → 404 on Rust backend

Sentry Evidence

GET: https://desktop-backend-hhibjajaja-uc.a.run.app/v1/users/me/subscription [404]
→ "Failed to load subscription: HTTP error: 404"

Other endpoints on the same Rust backend work fine (goals, transcription-preferences, etc. all return 200).

Verified

• curl https://api.omi.me/v1/users/me/subscription with valid auth → returns correct subscription JSON
• curl https://desktop-backend-hhibjajaja-uc.a.run.app/v1/users/me/subscription → 404
• curl https://desktop-api.omi.me/v1/users/me/subscription → 404

Affected Code

• Desktop client: APIClient.swift — getUserSubscription(), createCheckoutSession(), createCustomerPortalSession() all use baseURL (Rust backend)
• Error shown: SettingsPage.swift:5330 — catches any error and shows "Failed to load plan information."

Fix Options

1. Route subscription/payment calls to the Python backend — Add a mainBackendURL property to APIClient (defaulting to https://api.omi.me/, overridable via OMI_MAIN_BACKEND_URL env var) and use the customBaseURL parameter for these three calls
2. Add a proxy in the Rust backend — Forward /v1/users/me/subscription and /v1/payments/* to the Python backend
3. Add the subscription endpoint to the Rust backend — Implement it directly in Rust (most work)

Option 1 is the simplest and most targeted fix.

cc @kodjima33