v0.8.0-alpha.4 - Inventory ledger integrity

Inventory ledger integrity milestone

This prerelease records completion of INV-01, the inventory fact-chain foundation:

• transactional ledgers for order reserve, release, and confirmation
• stable event keys that make order stock retries idempotent
• before/after total, locked, and available inventory snapshots
• forward-compatible Flyway V18 migration and rollback boundary documentation
• MySQL Testcontainers coverage for invariants, concurrency, duplicate events, and transaction rollback

Pull request #63 was squash-merged as main commit 49f4369. The complete 14-job pull-request and post-merge main CI workflows passed.

Important boundary

This release does not deploy or execute inventory changes automatically. The production-like server remains on the previously deployed application images until a separately backed-up and verified upgrade is performed.