chore(backend): migrate to Spring Boot 3.5

[JuliusHenke]

Increment B7c: Spring Boot 2.7.18 → 3.5.16 (Spring 6.2, Security 6.5, Hibernate 6.6, Tomcat 10.1, Logback 1.5). Closes the open Tomcat/Spring/Security CVEs of the EOL 2.7 line; the interim version overrides are removed.

Two commits:

1. Boot 3.5, javax→jakarta, Hibernate 6, Logback 1.5, Flyway/SLF4J now Boot-managed, Ehcache 3.12.0 (jakarta classifier). The first commit alone does not compile, review them together.
2. springdoc 2.8.17 (springdoc-openapi-starter-webmvc-ui), Security 6 header lambdas, httptrace → httpexchanges, New Relic via io.micrometer:micrometer-registry-otlp (replaces micrometer-registry-new-relic and NewRelicConfig.kt).

Notable details

• Schema validation (ddl-auto: validate), HQL, the native query and id generation (B7a) all pass on Hibernate 6 without mapping changes.
• Test fixture fix: RelayDetailsRepositoryTest reused entities across tests; Hibernate 6 refuses to merge a detached entity whose row was deleted. Fresh instances per test.
• Logback 1.5: SizeAndTimeBasedRollingPolicy replaces SizeAndTimeBasedFNATP; %C{1.} → %C{1}; console appender uses an encoder.
• slf4j-api is now the Boot-managed 2.x (Logback 1.5 requires it).
• Removed database-platform (Hibernate 6 auto-detects).
• OTLP export is only enabled when NEW_RELIC_INGEST_KEY is set (tests cover both cases). Meter filters (plz_ignore_me, jvm.threads) are kept.

Deployment notes

• New Relic: metrics now go via OTLP to https://otlp.eu01.nr-data.net:4318/v1/metrics (EU region, delta temporality, api-key header). Same env var NEW_RELIC_INGEST_KEY. Not verified against a real account: check that metrics (e.g. jvm.memory.used) arrive and that dashboards/alerts still match; names may differ slightly (resource attribute host.name instead of host).
• Actuator: httptrace endpoint is now httpexchanges.
• Trailing slashes are no longer matched by Spring 6 (/relay/location/days/ → 404). The frontend does not use them.
• Take a DB backup before the first start; keep the previous image tag for rollback.
• Flyway 11.20 logs a warning that PostgreSQL 18 is newer than its tested version (17); migrations work.

Verification: ./gradlew test on JDK 17: 71 tests, 0 failures. spring-boot-properties-migrator reported no renamed properties (removed again). Not done: running the built image and a real New Relic export.

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