Move unit tests from tests/unit/ to internal/ following Go conventions:
- tests/unit/api/* -> internal/api/* (WebSocket handlers, helpers, duplicate detection)
- tests/unit/audit/* -> internal/audit/* (alert, sealed, verifier tests)
- tests/unit/auth/* -> internal/auth/* (API key, keychain, user manager)
- tests/unit/crypto/kms/* -> internal/auth/kms/* (cache, protocol tests)
Update import paths in test files to reflect new locations.
Benefits:
- Tests live alongside the code they test
- Easier navigation and maintenance
- Clearer package boundaries
- Follows standard Go project layout
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