remove forced Trakt source and canonicalize watched history ids

[neckaros]

Summary

This PR finishes the Trakt removal work for movie and TV history paths and replaces provider-specific watched/progress IDs with canonical RedSeat history IDs.

What changed

• Remove forced Trakt assumptions from movie and episode watched/progress reads and writes.
• Add canonical history IDs for:
  • movies: movie:imdb/<id> with fallback to TMDB then local RedSeat ID
  • series: series:<provider>/<id>
  • episodes: episode:<series-key>/<season>/<episode>
• Add a startup migration that rewrites existing watched/progress rows to the canonical IDs.
• Restrict that migration to Movies and Shows libraries only.
• Make the migration a no-op when there is no movie/episode history yet, so first install stays clean.
• Merge duplicate watched/progress rows safely during rewrite.
• Preserve existing episode external IDs on refresh by matching imported episodes back to stored season/episode pairs.

Why

Watched episode state had been tied to legacy provider-specific IDs, especially Trakt-derived ones. Once Trakt was removed from the core source flow, refreshed metadata could stop matching the old history rows and episodes appeared unwatched.

The fix is to store history against stable canonical IDs owned by RedSeat instead of whichever metadata provider happened to be used at the time.

Impact

• Existing installations migrate their movie and episode history on startup.
• New installations do nothing extra because there is no history to migrate.
• Movie and episode watched/progress state no longer depends on Trakt being configured in core.

Validation

• cargo check --message-format short