feat(api,domain): add batch commands endpoint and seed file tooling

[Wedvich]

Summary

• POST /api/sync/commands/batch — accepts a SeedFile (version + ordered command array) and processes each command through the existing dispatch pipeline within a single transaction. All-or-nothing atomicity: if any command fails, the entire batch rolls back with the index and reason.
• snapshotsToSeedFile() in @heim/domain — converts bootstrap snapshots into a re-importable seed file. Built-in converters for ProductType and StockItem, extensible via registerSeedConverter(). Tenant/User always excluded. Output sorted by dependency priority (ProductType before StockItem).
• Batch endpoint reuses existing loadStreamEvents, commandRegistry.handle(), appendEvents, and projectorRegistry.apply() — no new DB setup or duplicated wiring.

Test plan

• Domain: 10 seed tests — conversion, ordering, filtering, exclusions, edge cases
• API: 6 batch endpoint tests — auth, validation (empty/unsupported version), unknown stream type, success with multiple commands, rollback on rejection
• yarn turbo typecheck — clean
• yarn turbo test — 84 domain + 118 API tests pass
• yarn turbo lint — clean

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