[Synced Transcripts] Add timing fields to transcript entries

[sztomek]

Description

This PR is part of the Radical Speed Month initiative.
It lays the groundwork for synced transcripts by threading timing data through the transcript pipeline. TranscriptEntry.Text gains startTimeMs/endTimeMs fields (defaulting to -1L so all existing callers are unaffected). All parsers (WebVTT, SRT, JSON) now extract timing from their source formats, and the joinSplitSentences() sanitizer accumulates timing spans when merging fragments.
Also adds the SYNCED_TRANSCRIPTS feature flag (Free tier, debug/prototype default, Firebase remote + dev toggle).

Testing Instructions

Just review the code please.

Checklist

• If this is a user-facing change, I have added an entry in CHANGELOG.md
• Ensure the linter passes (./gradlew spotlessApply to automatically apply formatting/linting)
• I have considered whether it makes sense to add tests for my changes
• All strings that need to be localized are in modules/services/localization/src/main/res/values/strings.xml
• Any jetpack compose components I added or changed are covered by compose previews
• I have updated (or requested that someone edit) the spreadsheet to reflect any new or changed analytics.

[Copilot]

Pull request overview

This PR lays groundwork for synced transcripts by adding start/end timing (ms) to transcript text entries and threading that timing through subtitle parsers and transcript sanitization, gated behind a new feature flag.

Changes:

• Added startTimeMs / endTimeMs fields (default -1L) to TranscriptEntry.Text.
• Updated WebVTT/SRT parsing to attach cue timing to TranscriptEntry.Text, and JSON parsing to convert startTime/endTime into ms.
• Updated transcript sanitization to accumulate timing spans when joining split sentences; added a new SYNCED_TRANSCRIPTS feature flag.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
modules/services/utils/src/main/java/au/com/shiftyjelly/pocketcasts/utils/featureflag/Feature.kt	Adds the SYNCED_TRANSCRIPTS feature flag definition.
modules/services/model/src/main/java/au/com/shiftyjelly/pocketcasts/models/to/TranscriptEntry.kt	Extends TranscriptEntry.Text with startTimeMs/endTimeMs (default -1L).
modules/services/repositories/src/main/java/au/com/shiftyjelly/pocketcasts/repositories/transcript/TranscriptParser.kt	Threads timing through subtitle parsing and JSON parsing into transcript entries.
modules/services/repositories/src/main/java/au/com/shiftyjelly/pocketcasts/repositories/transcript/TranscripSanitization.kt	Accumulates and propagates timing when joining sentence fragments.
modules/services/repositories/src/test/java/au/com/shiftyjelly/pocketcasts/repositories/transcript/WebVttParserTest.kt	Updates expected entries to include parsed WebVTT timings.
modules/services/repositories/src/test/java/au/com/shiftyjelly/pocketcasts/repositories/transcript/SrtParserTest.kt	Updates expected entries to include parsed SRT timings.

[dangermattic]

	1 Warning
⚠️	This PR is assigned to the milestone 8.13. The due date for this milestone has already passed. Please assign it to a milestone with a later deadline or check whether the release for this milestone has already been finished.

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