feat: 가계부·게시판 백엔드 API 구현

- com.sb.web 패키지로 재구성: account / auth / board / user / admin / common
- 가계부(account): 내역(필터), 계좌·순자산, 예산, 분류, 정기 거래, 투자 포트폴리오
  (종목·매매 이력, 이동평균 평단·실현/평가손익)
- MyBatis + MariaDB + Redis 세션, BCrypt
- 스키마 자동 초기화(db/*.sql, 멱등), 사용자별 데이터 격리(member_id)
- 문서: docs/BACKEND.md, .env.example

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

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

@@ -0,0 +1,51 @@
+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.SessionUser;
+import com.sb.web.auth.dto.SignupRequest;
+import com.sb.web.auth.service.AuthService;
+import com.sb.web.auth.web.AuthInterceptor;
+import jakarta.servlet.http.HttpServletRequest;
+import jakarta.validation.Valid;
+import lombok.RequiredArgsConstructor;
+import org.springframework.http.HttpStatus;
+import org.springframework.http.ResponseEntity;
+import org.springframework.web.bind.annotation.*;
+
+/**
+ * 인증 API.
+ *   POST /api/auth/signup  회원가입 (비보호)
+ *   POST /api/auth/login   로그인   (비보호)
+ *   POST /api/auth/logout  로그아웃 (보호)
+ *   GET  /api/auth/me      내 정보  (보호)
+ */
+@RestController
+@RequestMapping("/api/auth")
+@RequiredArgsConstructor
+public class AuthController {
+
+    private final AuthService authService;
+
+    @PostMapping("/signup")
+    public ResponseEntity<MemberResponse> signup(@Valid @RequestBody SignupRequest req) {
+        return ResponseEntity.status(HttpStatus.CREATED).body(authService.signup(req));
+    }
+
+    @PostMapping("/login")
+    public LoginResponse login(@Valid @RequestBody LoginRequest req) {
+        return authService.login(req);
+    }
+
+    @PostMapping("/logout")
+    public ResponseEntity<Void> logout(HttpServletRequest request) {
+        authService.logout(AuthInterceptor.resolveToken(request));
+        return ResponseEntity.noContent().build();
+    }
+
+    @GetMapping("/me")
+    public SessionUser me(@RequestAttribute(AuthInterceptor.CURRENT_MEMBER) SessionUser current) {
+        return current;
+    }
+}