chore: add createMany use case (Batch accounts 3/5)

[gantunesr]

Stack part 3 of 5 split from #601. Adds AccountUseCases.createMany, bounded account creation concurrency, deduping/idempotency, bulk insertion, and mutation serialization. No public keyring RPC is wired in this part.

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Note

Medium Risk
Touches core account creation/deletion flows by introducing mutation serialization and changing create to emit account-created events even when reusing an existing account; concurrency/bulk-insert paths may surface race or ordering issues.

Overview
Adds AccountUseCases.createMany to create multiple accounts in one call, deduping duplicate derivation paths, reusing existing accounts, creating missing ones with a small concurrency cap, and persisting via insertMany while scheduling background full scans only for newly-created/synchronized accounts.

Refactors derivation-path handling into shared helpers and introduces a simple mutation queue (#runAccountMutation) to serialize account mutations; create and delete now run under this queue, and create now emits emitAccountCreatedEvent even when the account already exists.

Updates unit tests to cover createMany behaviors (reuse, dedupe, error propagation, in-flight create ordering) and documents the new use case in the changelog.

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