feat: add Czech Republic (CZ) DPH tax regime

[pablocaeg]

Summary

Added the Czech Republic DPH (VAT) tax regime. Rebuilt on v0.400.0 using the new rules package.

• Two current rate tiers since the January 2024 consolidation reform:
  • Standard (Základní sazba) at 21%
  • Reduced (Snížená sazba) at 12%, merged from the previous 15% first reduced and 10% second reduced
• Historical VAT rates back to 2004 (EU accession)
• DIČ (Daňové identifikační číslo) validation with three format types:
  • 8-digit legal entities using modulo-11 checksum with weights [8,7,6,5,4,3,2]
  • 10-digit individuals using Rodné číslo divisibility by 11 check
  • 9-digit special identifiers
• IČO (Identifikační číslo osoby) business registration identity with shared modulo-11 validation
• Supplier must have either DIČ or IČO
• Domestic B2B example invoice
• 29 tests, 93.7% code coverage

Sources

• Czech Republic - General rules and VAT rates (portal.gov.cz)
• Registering for VAT in the Czech Republic (portal.gov.cz)
• python-stdnum Czech DIČ reference

Pre-Review Checklist

• Opened this PR as a draft
• Read the CONTRIBUTING.md guide
• Performed a self-review of my code
• Added thorough tests with at least 90% code coverage
• Modified or created example GOBL documents to show my changes in use
• Added links to the source of the changes in tax regimes or addons
• Run go generate . to ensure that the Schemas and Regime data are up to date
• Reviewed and fixed all linter warnings
• Been obsessive with pointer nil checks to avoid panics
• Updated the CHANGELOG.md with an overview of my changes

And if you are part of the org:

• Requested a review from Copilot and fixed or dismissed (with a reason) all the feedback raised.
• Requested a review from @samlown.

[claude]

Claude Code Review

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