feat: support unified mixed MDBList catalogs by username/slug

[nobnobz]

Fixes #415

Summary

This adds proper support for mixed MDBList dynamic lists queried through the unified items endpoint.

MDBList can expose mixed lists as separate movie/show metadata entries for the same public username/slug, while the actual mixed content is available from /lists/{username}/{slug}/items?unified=true.

This change adds a unified import and resolution path so mixed MDBList lists can be consumed as a single all catalog instead of being split into separate movie and series catalogs.

What changed

• group MDBList sibling list metadata entries by username/slug during import flows
• add a synthetic internal catalog id for unified mixed catalogs: mdblist.{username}.{slug}.unified
• fetch mixed MDBList items through the unified items endpoint
• preserve per-item mediatype when normalizing the mixed response
• keep response order stable for unified mixed catalogs instead of inventing synthetic popularity values
• add UI support to choose between one unified mixed catalog or separate movie and series catalogs
• keep existing movie-only and series-only MDBList flows unchanged
• ignore local generated addon runtime data so SQLite/cache files do not get committed accidentally

Notes

• MDBList does not expose a dedicated unified numeric list id for these mixed lists, so the addon uses a synthetic internal identity.
• Unified mixed ordering preserves the original API response order. If an explicit internal order is needed, it is derived from response position only.

Validation

• node --test --test-force-exit addon/tests/mdbListUnified.test.js
• frontend build verified locally
• backend build verified locally
• local/live smoke checks against MDBList mixed list responses during development