feat: API endpoint cache

[netroms]

PR: Entity-Type Granularity for ETag Caching

Summary

• Implements entity-type-specific version counters for ETag caching instead of a single global version
• Changes to one entity type (e.g., OrganisationUnit) no longer invalidate caches for other entity types (e.g., DataSet)
• Redis keys: etag:version:OrganisationUnit, etag:version:DataSet, etc.

Test plan

• Unit tests pass (19 tests in ConditionalETagServiceTest)
• Verify /api/organisationUnits returns 304 when only DataSet changes
• Verify /api/organisationUnits returns 200 with new data when OrganisationUnit changes
• Test in clustered environment with Redis pub/sub

[sonarqubecloud]

Quality Gate failed

Failed conditions
2 New issues
2 New Code Smells (required ≤ 0)
1 New Critical Issues (required ≤ 0)

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud