fix(playback): restore SDR alternate fallback for burn-in subtitles

[cursor]

Problem

Follow-up to #617.

Selecting a bitmap (PGS/VOBSUB) subtitle on an HDR source that has an SDR alternate returned adaptation_unavailable with subtitle_conversion_unsupported instead of falling back to the SDR version and burning the subtitle in.

Concrete trigger:

1. Title has HDR10 4K + SDR 1080p versions.
2. Start plays the HDR file directly (client claims HDR10).
3. User selects an embedded PGS track (no bitmap renderer → burn-in required).
4. Replan terminals instead of adopting the SDR alternate.

Approach

#617 correctly renamed burn-in-only HDR/4K refusals to subtitle_conversion_unsupported so the UI can name the subtitle. terminalAllowsAlternateFileV3 was not updated and still only treated hdr_transcode_unsupported / no_alternate_version as version-shaped blockers, so the existing alternate-file path never ran.

Allow subtitle_conversion_unsupported through the same gate. Alternate selection remains opt-in (quality != original, track/quality/output/failure replan), remaps the subtitle identity, and only adopts the alternate when that plan succeeds. If no alternate works, the subtitle-named terminal still surfaces.

Testing

$ go test ./internal/api/handlers/ -run 'TestTerminalAllowsAlternateFileV3IncludesHDRIncompatibility|TestHandleReplanPlaybackV3BitmapSubtitleFallsBackFromHDRToSDRVersion|Alternate|BitmapSubtitle' -count=1
ok  	github.com/Silo-Server/silo-server/internal/api/handlers

$ go test ./internal/playback/ -run 'NamesTheSubtitle|KeepsTheHDR|DisabledTranscodeStillNames' -count=1
ok  	github.com/Silo-Server/silo-server/internal/playback

$ gofmt -l internal/api/handlers/playback_v3.go internal/api/handlers/playback_v3_test.go
# clean

TestHandleReplanPlaybackV3BitmapSubtitleFallsBackFromHDRToSDRVersion failed on pristine ee9356aa (#617) and passes with this change.

AI Disclosure

• Tool(s): Cursor
• Model(s): cursor-grok-4.5-high
• Involvement: fully AI-generated
• Adversarial review: Considered whether every subtitle_conversion_unsupported should trigger an alternate attempt. Trying once is safe: remapping can fail closed with subtitle_unavailable_in_version, a failed alternate still surfaces the subtitle-named terminal, and quality=original / seek-only replans remain pinned. Did not broaden planner reason naming from fix(playback): claim Safari DV/HDR10 decode evidence and preserve dvvC in remuxes #617.

Checklist

• I ran an adversarial AI review of the diff and summarized findings above.
• I ran the repo verify commands relevant to this change: gofmt and targeted go test for the touched packages. Full make lint / web lint were not required (backend-only, two files).