Add optional preview_start_time parameter to mix enhance and mastering endpoints

[daveronan]

Summary

• Add optional preview_start_time: Optional[float] parameter to MixEnhanceRequest and MasteringRequest models
• When provided, include previewStartTime in the API request payload under mixReviveData or masteringData; when omitted, existing RMS-based auto-selection is preserved (backward compatible)
• Parse preview_start_time from retrieve response objects (already implemented in EnhancedTrackResult and PreviewMasterResult)
• Update docstrings, README examples (also fixes pre-existing incorrect model names and dict access in the enhancement section), and add unit tests covering provided, zero, and omitted cases

Staging verification

All 5 test cases passed on tonn-staging.roexaudio.com:

Input	Returned preview_start_time
previewStartTime: 45.0	45.0
No previewStartTime	105.0 (RMS auto)
previewStartTime: 30.0	30.0
previewStartTime: 9999.0	123.67 (clamped)
previewStartTime: 0	0.0

Test plan

• Unit tests pass (32/32)
• Staging integration tests pass (5/5)
• Verify backward compatibility with existing client code that doesn't use the new parameter

Made with Cursor

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Note

Low Risk
Low risk additive change: introduces an optional request field that only affects preview segment selection and is omitted by default; main risk is API payload compatibility if backend rejects the new previewStartTime key.

Overview
Adds an optional preview_start_time field to MasteringRequest and MixEnhanceRequest and forwards it as previewStartTime in the /masteringpreview and /mixenhance* request payloads when provided (including 0.0), preserving existing RMS-based auto-selection when omitted.

Updates docs/README examples to show the new parameter and correct typed result access for preview/final URLs, and adds unit tests covering provided/zero/omitted behavior for both controllers.

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