feat: support multiple Radarr and Sonarr instances

[helitra]

This PR extends SeerrFin’s existing Servarr integration from one Radarr and one Sonarr instance to multiple instances per type, without changing the core request flow.

The goal is not to redesign the plugin, but to let users register additional Radarr/Sonarr endpoints and have the plugin treat them the same way as the current single-instance setup.

What changed

• Added a multi-instance Servarr configuration model.
• Kept legacy single-instance fields for backward compatibility.
• Migrated existing Radarr/Sonarr settings into the new list-based structure automatically.
• Reworked the admin configuration UI to allow adding multiple Radarr and Sonarr instances.
• Added a + flow for creating additional instances.
• Added per-instance defaults and a Radarr-only 4K flag.
• Updated Requests progress handling to resolve the correct instance per request.
• Updated open links in the Requests UI to use the correct Radarr/Sonarr instance.
• Preserved the existing Seerr request flow and quality-profile behavior.

Compatibility notes

• Existing single-instance installs continue to work.
• Legacy Radarr/Sonarr values are still supported and are migrated into the new structure.
• Seerr remains a single instance; only Radarr and Sonarr were expanded.

Testing

Verified locally and in Jellyfin using a separate test manifest and fork release:

• repository added successfully
• plugin installed successfully
• Jellyfin restart and setup successful
• single-instance configs still work
• multiple Radarr/Sonarr instances can be added and saved
• Requests/Progress/Quality Profile data loads correctly
• the correct target instance is used for the request flow

Test release used

• 1.6.5.2
• separate test manifest repo: helitra/SeerrFin-test-manifest
• test manifest URL:
  • https://raw.githubusercontent.com/helitra/SeerrFin-test-manifest/main/manifest.json

[helitra]

[varunaditya-plus]

Thank you for your contribution. It's promising, but the routing is not safe enough to merge yet:

• Map instances using Seerr’s serverId/serviceId, not TMDB or instance-local IDs.
• Preserve the backend-provided openUrl.
• Fix 4K setting persistence and support Sonarr 4K.
• Cache instance snapshots to avoid querying every server every 10 seconds.
• Test duplicate titles/IDs and secondary instances.

Until then, this doesn't fully address #22.