docs: align README with concurrency model and testing strategy

[heitorrsdev]

Description

This PR updates the README to reflect the current architectural state of the project after the introduction of database-enforced concurrency guarantees and the formalization of the testing strategy.

It removes outdated sections and ensures that the documentation accurately represents the system’s integrity model, layered architecture, and SQL-first approach.

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What was done

• Rewrote README in English to align with project standards
• Removed the Current Status section to avoid documentation drift
• Updated architecture description to reflect enforced dependency rules
• Clarified database-level invariant enforcement strategy
• Documented concurrency validation approach at a high level
• Linked explicitly to testing.md for detailed concurrency and integration testing strategy
• Ensured README focuses on stable architectural foundations rather than transient implementation state

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Why

• The previous README no longer reflected the current concurrency model and testing guarantees
• The Current Status section introduced maintenance overhead and risk of desynchronization
• Documentation should emphasize architectural invariants and long-term design decisions, not temporary development state
• A stable, principle-driven README reduces churn and improves long-term clarity

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Notes

• This PR does not introduce functional changes
• All behavioral and concurrency guarantees are already covered by integration tests
• Detailed architectural decisions remain documented in docs/adr