feat(scheduler): Allow bypassing queue logic via CardAnswer flag

[xuanyue202]

Problem

Currently, the backend scheduler assumes all card answers processed via the answer_card endpoint originate from the standard review queue (from_queue is implicitly true). This doesn't accurately represent scenarios like card previews or potentially some custom study sessions where answering a card should not affect the main queue counts or state.

Solution

This PR introduces an optional boolean field from_queue to the CardAnswer protobuf message definition (proto/anki/scheduler.proto).

When from_queue is set to false by the client, the backend service (rslib/src/scheduler/service/answering.rs) will now set the internal from_queue flag to false for that specific answer. This allows the scheduler to correctly handle answers that should not modify the standard queue state.

If from_queue is omitted or set to true, the behavior remains unchanged (the answer is considered from_queue = true).

Impact

This change provides more flexibility for clients interacting with the scheduler, enabling them to distinguish between answers that are part of the normal review flow and those that occur outside of it (e.g., previews), ensuring queue metrics remain accurate.

[abdnh]

LGTM

[dae]

Reverting this due to #3983

If a separate method proves necessary in the future, we should address this issue at the same time.