Open transcription model identifiers

[atacan]

What changed

• add OpenTranscriptionModelIdentifier as an unrestricted wire string
• retain TranscriptionModelIdentifier as the existing closed enum
• use the open identifier for model-list responses and transcription request fields across OpenAPI and AsyncAPI
• simplify the translation model schema and correct the model-list example
• bump the API document version to 0.6.0

Why

The database can expose model identifiers newer than a generated SDK. Wire-facing identifiers must therefore round-trip as opaque strings while the server can continue using its enum for routing and provider conversions.

Impact

Newly generated SDKs accept model IDs discovered from /speech-to-text-models without enum decoding failures. JSON and query parameter representations remain unchanged.

Validation

• regenerated Swift OpenAPI server and type sources in the companion server branch
• verified generated wire-facing model properties are strings
• verified the existing transcription model enum remains unchanged