Split reviewer backend protobuf RPCs

[andrewsanchez]

Source split from ankitects#4289.

Breakdown sections covered:

• 5. NextCardData RPC
• 7. ReviewerAction RPC
• 8. PlayAVTags audio RPC
• 28. Custom scheduling backend/frontend RPC shape
• 34. Partial template stripping
• 39. get_config_string / set_config_string

Source fidelity:

• This PR copies exact final file contents from PR Feat/Svelte port for reviewer ankitects/anki#4289.
• No implementation changes were made while splitting.
• Commit author is preserved as Luc Mcgrady.

Follow-up notes from the breakdown:

• NextCardData field numbers and nested message structure still need final review.
• A dedicated reviewer preferences endpoint may be preferable to config/meta access.
• Reviewer storage size may need limits because config-string data syncs.