Re-enables Spring Security's CSRF filter (was disabled with a TODO comment).
Uses CookieCsrfTokenRepository so the frontend can read the XSRF-TOKEN
cookie and send it as X-XSRF-TOKEN on state-mutating requests.
Returns CSRF_TOKEN_MISSING error code on 403 instead of generic FORBIDDEN.
Updates all WebMvcTest classes to include .with(csrf()) on POST/PUT/PATCH/
DELETE/multipart requests, and fixes integration tests to supply the
XSRF-TOKEN cookie + header directly (lazy generation in Spring Security 7).
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>