fix(backend): switch to sentry-spring-boot-4:8.41.0 for Spring Boot 4/SF7 compatibility

sentry-spring-boot-starter-jakarta 8.5.0 does not support Spring Boot 4.0 —
it logs an "Incompatible Spring Boot Version" warning and its SentryAutoConfiguration
crashes SF7 bean-name generation. sentry-spring-boot-4 (added in 8.21.0) is the
dedicated Spring Boot 4 module with a fixed auto-configuration class.

- Replace sentry-spring-boot-starter-jakarta:8.5.0 with sentry-spring-boot-4:8.41.0
- Delete SentryConfig.java — workaround no longer needed, auto-config handles init
- Remove spring.autoconfigure.exclude from application.yaml + application-test.yaml
- Delete SentryConfigTest.java — tested the deleted workaround class
- Update ApplicationContextTest: assert Sentry.isEnabled() is false when no DSN set

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/src/main/resources/application.yaml

@@ -38,13 +38,6 @@ spring:
           starttls:
             enable: true
 
-  autoconfigure:
-    exclude:
-      # SentryAutoConfiguration fails on Spring Boot 4/Spring Framework 7: Spring cannot generate a
-      # bean name for the triply-nested SentryAutoConfiguration$HubConfiguration$SentrySpanRestClientConfiguration.
-      # Sentry is initialized manually via SentryConfig instead. See #580.
-      - io.sentry.spring.boot.jakarta.SentryAutoConfiguration
-
 server:
   # Behind Caddy/reverse proxy: trust X-Forwarded-{Proto,For,Host} so that
   # request.getScheme(), redirect URLs, and Spring Session "Secure" cookies