fix(server): load embedded Plex subtitles

[KyleTryon]

Embedded text subtitles detected in Plex could fail to load in the editor. Prepare a Plex playback decision, then extract the selected track as SRT through Plex's subtitle endpoint using a separate session from HLS playback. Preserve the selected media version, part, and cue timestamps.

Cancel upstream requests when the browser disconnects, reject failed decisions or changed subtitle selections, and display actionable errors in the editor. Reuse existing helpers and stable media handles; no new dependencies. Embedded tracks still need to be selected in Plex first.

Validation:

• pnpm preflight passed: formatting, lint, type checks, unused-code checks, and all 476 tests.
• Regression coverage includes decision/extraction ordering, session isolation, SRT/ASS handling, sidecars, media selection, cancellation, malformed responses, and frontend error messages.
• Verified extraction with a disposable Plex server and synthetic embedded SRT/ASS fixtures, including language changes and preserved timestamps.

[Copilot]

🟢 Approval recommended

The implementation is coherent, preserves existing proxy invariants, and includes focused regression coverage for the new behavior.

Pull request overview

Adds reliable Plex embedded subtitle extraction and actionable editor errors.

Changes:

• Prepares and validates Plex subtitle decisions using isolated sessions.
• Preserves media selection and supports cancellation.
• Parses server error details for frontend subtitle failures.

File summaries

File	Description
apps/server/src/providers/types.ts	Adds subtitle stream metadata.
apps/server/src/providers/shared/mediaProxy.ts	Normalizes the new metadata.
apps/server/src/providers/plex/playback.ts	Implements decision-driven subtitle extraction.
apps/server/src/providers/plexPlayback.test.ts	Adds Plex subtitle regression coverage.
apps/frontend/src/api/cliparrClient.ts	Exposes JSON error parsing.
apps/frontend/src/api/cliparrClient.test.ts	Tests error parsing and cancellation.
apps/frontend/src/components/editor/useSubtitleCues.ts	Displays actionable subtitle errors.

Review details

• Files reviewed: 7/7 changed files
• Comments generated: 0
• Review effort level: Balanced

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