Spring Boot 4 migration — blocks Springdoc 3 (#152)

[amavashev]

Tracks the Spring Boot 3.x → 4.x migration in `cycles-server-admin`. Currently blocks #152 and is the peer of `runcycles/cycles-server#134`.

Why this isn't a Dependabot merge

Springdoc 3.x is built against Spring Boot 4 and pulls in SB4 autoconfigs that conflict with this repo's SB3 baseline:

```
BeanDefinitionOverrideException: Invalid bean definition with name 'conventionErrorViewResolver'

• defined in: org.springframework.boot.webmvc.autoconfigure.error.ErrorMvcAutoConfiguration (SB4 path)
• already bound: org.springframework.boot.autoconfigure.web.servlet.error.ErrorMvcAutoConfiguration (SB3 path)
  ```

SB4 moved `ErrorMvcAutoConfiguration` from `autoconfigure.web.servlet` → `webmvc.autoconfigure`. Both class paths exist on the classpath simultaneously when SB4 transitives meet an SB3 base, hence the override exception. Springdoc 3 cannot merge until SB4 lands.

Scope of the migration

• Bump `spring-boot-starter-parent` 3.5.x → 4.0.x
• Java baseline: confirm 21 satisfies SB4 minimum (currently 17 baseline; should be fine)
• Servlet 6 / Jakarta EE 10 surface check
• Project Loom virtual-thread defaults (SB4 enables them in some configs by default)
• Re-test all admin endpoints, OpenAPI contract diff, integration tests
• Coordinate with `cycles-server` (chore(ci): auto-merge Dependabot patch-level bumps #134) and `cycles-server-events` so all three flip together; they share the Spring Boot baseline.

Cross-repo

• `cycles-server` peer: chore(deps): bump org.springframework.boot:spring-boot-starter-parent from 3.5.14 to 4.0.6 in /cycles-protocol-service cycles-server#134 (also blocked, also a Dependabot SB4 bump)
• `cycles-server-events` will need a companion bump when the time comes — currently still on SB 3.5.14.

Recommend doing all three in coordinated PRs so we don't have an SB3 ↔ SB4 mixed-version run in production.

Reference

• PR chore(deps): bump org.springdoc:springdoc-openapi-starter-webmvc-ui from 2.8.16 to 3.0.3 in /cycles-admin-service #152 (closed): chore(deps): bump org.springdoc:springdoc-openapi-starter-webmvc-ui from 2.8.16 to 3.0.3 in /cycles-admin-service #152