fix(tags): preserve Matroska LanguageBCP47 regions

[maghuro]

Summary

Preserves explicit Matroska LanguageBCP47 / LanguageIETF regional variants before they are lost by Jellyfin/FFmpeg, allowing the existing region-aware language flag resolver to receive values such as pt-BR, pt-PT, en-US, and en-GB.

Fixes #789.

Root cause

Matroska can contain both a legacy Language element (por, eng, etc.) and an authoritative BCP-47 language element (pt-BR, en-US, etc.).

For the tested MKVToolNix files, FFmpeg/Jellyfin exposes only the legacy/base value through MediaStream.Language. As a result, Jellyfin Enhanced never receives the regional tag even though its client-side flag resolver already understands it.

Implementation

• Adds a lightweight internal Matroska/EBML reader for local Matroska/WebM sources.
• Reads only the container metadata required to locate Tracks / TrackEntry; it does not scan media clusters.
• Prefers LanguageBCP47 over the legacy Language element.
• Falls back to Jellyfin's existing MediaStream.Language for unsupported, inaccessible, malformed, non-Matroska, or non-regional media.
• Aligns Matroska languages only with embedded audio streams and only when the embedded audio-track counts match exactly, avoiding incorrect ordinal assignment when external or missing streams are present.
• Uses the resolved language in both server-side tag-cache extraction and the /tag-data path used when the server cache is disabled.
• Updates item-details and Series/Season first-episode language paths to use the same region-aware projection.
• Bumps the tag-cache schema from v2 to v3 so stale entries containing region-less language values are discarded and rebuilt.
• Adds no external executable or package dependency.

Bare-language policy is intentionally unchanged. For example, an explicit en-US can resolve to the US flag, while bare eng continues to follow Jellyfin Enhanced's existing default behavior.

Testing

Runtime-tested on:

• Jellyfin Server 12.0.0
• Jellyfin Enhanced 12.4.1.0
• Docker / Jellyfin Web
• Server-Side Tag Cache enabled and disabled

Verified with real Matroska files containing:

• por + pt-PT
• por + pt-BR
• eng + en-US
• eng + en-GB

Verified regional values in the rebuilt v3 server tag cache, including pt-PT, pt-BR, en-US, en-GB, fr-CA, and es-419.

Also verified the non-cache card path and item-details path, including Series/Season first-episode language resolution.

Build/static validation:

• Jellyfin 12 target / .NET 10: 0 warnings, 0 errors
• Jellyfin 10.11 target / .NET 9: 0 warnings, 0 errors
• node --check passed for both modified JavaScript files
• git diff --check passed

AI assistance

AI assistance was used during investigation and implementation; I reviewed and tested the changes and understand the implementation.