Prevent lost updates with optimistic locking

[Pan14ek]

Purpose

Prevent silent lost updates across mutable backend aggregates by requiring an ETag-based version precondition on mutations.

Type

• Bug fix

Changes

• Add JPA version columns and API-visible versions for Event, Task, Lobby, User, Plan, and notification preferences.
• Require quoted If-Match headers for PATCH, Plan PUT, deletes, and lobby member removal; map missing, malformed, and stale conditions to 428, 400, and 409.
• Persist default notification preferences on first GET so clients receive an initial ETag.
• Add unit coverage for stale Event and Task mutations and precondition parsing.

Expected result

Concurrent stale writes no longer silently overwrite a newer mutation; the stale request receives an RFC 7807 409 response.

How to use and test

1. Read a resource representation and retain its version or ETag.
2. Send the quoted version in the If-Match request header when mutating it.
3. Retry after a fresh read when a stale request returns 409.
4. Run ./gradlew test.

Verification

• ./gradlew test passed before the final focused test additions.
• Checkstyle was run after resolving parameter-count issues, but the final combined quality gate was not completed.
• PostgreSQL/Testcontainers race coverage for CONC-T01-T04 remains required.

Metrics

All fitness and Sonar metrics: not measured.

[sonarqubecloud]

Quality Gate failed

Failed conditions
64.7% Coverage on New Code (required ≥ 80%)

See analysis details on SonarQube Cloud