feat: track episode availability from media servers scanners

[0xSysR3ll]

Description

Part 3 of #1671, stacked on #3355.

Wires episode availability into the media server scanners so media-server-only users get per-episode badges without Sonarr.
Episode numbers come from the library agent, so Seerr's metadata provider must match that agent (TMDB or TVDB).

Sonarr tracking stays TVDB-only.

How Has This Been Tested?

Only unit tests for now.

Screenshots / Logs (if applicable)

Checklist:

• I have read and followed the contribution guidelines.
• Disclosed any use of AI (see our policy)
• I have updated the documentation accordingly.
• All new and existing tests passed.
• Successful build pnpm build
• Translation keys pnpm i18n:extract
• Database migration (if required)

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[github-actions]

This pull request has merge conflicts. Please resolve the conflicts so the PR can be successfully reviewed and merged.