feat: 비밀번호 변경 기능 (PUT /api/auth/password)

현재 비번 검증 후 BCrypt 교체. 소셜 계정/동일 비번 거부.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/main/java/com/sb/web/auth/controller/AuthController.java

@@ -3,6 +3,7 @@ package com.sb.web.auth.controller;
 import com.sb.web.auth.dto.LoginRequest;
 import com.sb.web.auth.dto.LoginResponse;
 import com.sb.web.auth.dto.MemberResponse;
+import com.sb.web.auth.dto.PasswordChangeRequest;
 import com.sb.web.auth.dto.SessionUser;
 import com.sb.web.auth.dto.SignupRequest;
 import com.sb.web.auth.service.AuthService;
@@ -48,4 +49,12 @@ public class AuthController {
     public SessionUser me(@RequestAttribute(AuthInterceptor.CURRENT_MEMBER) SessionUser current) {
         return current;
     }
+
+    @PutMapping("/password")
+    public ResponseEntity<Void> changePassword(
+            @Valid @RequestBody PasswordChangeRequest req,
+            @RequestAttribute(AuthInterceptor.CURRENT_MEMBER) SessionUser current) {
+        authService.changePassword(current.getId(), req);
+        return ResponseEntity.noContent().build();
+    }
 }