fix: serialize SQLite workflow writes

[cmilesio]

What

Serialize transactional workflow writes within each SQLite-backed store. Apply the same lock to chain advancement, chain failure, batch creation, batch settlement, and pruning so every workflow transaction follows one SQLite write policy.

Why

The SQLite 1.58 migration passed PR CI, but two consecutive merge-commit runs exposed SQLITE_BUSY during 32-way terminal batch settlement under the race detector. SQLite permits one writer, and competing deferred transactions can still be rejected after the configured busy timeout. Per-store serialization reflects that database constraint and preserves successful concurrent queue behavior without changing public APIs, configuration, persisted data, the minimum Go version, or non-SQLite drivers.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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