ISSUE-46: fix notification SSE buffering

[dongmucat]

Summary

• Prevent notification SSE requests from passing through ContentCachingResponseWrapper in RequestLoggingFilter.
• Set text/event-stream, Cache-Control: no-cache, no-transform, and X-Accel-Buffering: no for the notification SSE response.
• Add regression coverage proving SSE uses the raw response while normal API requests keep cached logging behavior.

Backend evidence

• The notification manager already registers emitters and sends connected/notification events.
• Reproduction test failed before the fix because /api/web/notifications/sse reached the filter chain as a ContentCachingResponseWrapper, which can buffer streaming bytes.
• The same test passes after bypassing the wrapper for notification SSE.

Tests

• cd server && JDK_JAVA_OPTIONS="-XX:+EnableDynamicAgentLoading" ./mvnw -pl skillhub-app -am test -Dtest=RequestLoggingFilterTest -Dsurefire.failIfNoSpecifiedTests=false
• make test-backend-app

Fixes #519