feat: add per-turn automatic translation delivery

[Gwen317]

Summary

Implements the confirmed design for Issue #176:

• Capture tts_enabled and delivery_enabled independently for each target language at Turn start.
• Persist each Final Turn before scheduling delivery; one enabled channel creates one asynchronous message.
• Pin one verified destination_ref per channel and use auto:final_turn:{turn_id}:{channel}:{destination_ref} for idempotency.
• Keep message snapshots immutable and include both source text and translated text.
• Allow both email and WeCom through the existing outbox, queue, worker, retry, and provider path.
• Add migration, contracts, focused tests, and the short design proposal.

Verification

• go test -count=1 -race ./... in packages/contracts
• go test -count=1 -race ./... in services/api
• go test -count=1 -race ./... in services/realtime-audio

Refs #176

[fennoai]

Review summary:

Found one hard persistence bug and one wiring gap in the new automatic-delivery path.

[fennoai]

The parameter order here is wrong: $3 is bound from preference.Enabled, but the third column is destination_ref. This will try to store a bool into the destination-ref column and swap the enabled/destination values for every preference write.

[fennoai]

This adapter is only useful if the realtime runtime actually uses it. In this repo the manager still consumes session.LanguageConfigReader, and there is no call site for NewRealtimeAudioLanguageConfigReader, so OutputRoutes never reach the turn opener in production.