feat(backend): implement domain — users, projects, time entries, reports

- User, Project, TimeEntry JPA entities with SLF4J logging in services
- Native SQL aggregate queries: per-day (weekly) and per-project (monthly)
  both using COALESCE(project_rate, user_rate) for cost calculation
- ReportService parses ISO week/month strings, fills zero-day gaps,
  computes timeBudgetPct / costBudgetPct / breached per project
- ProjectService piggybacks on aggregateAllByProject for budget status
  on GET /api/projects (no N+1)
- GlobalExceptionHandler maps validation errors → 400 ProblemDetail,
  EntityNotFoundException → 404, IllegalArgumentException → 400
- Lombok on pom.xml (optional, used for @Getter/@Setter/@NoArgsConstructor)

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/pom.xml

@@ -46,6 +46,11 @@
             <artifactId>postgresql</artifactId>
             <scope>runtime</scope>
         </dependency>
+        <dependency>
+            <groupId>org.projectlombok</groupId>
+            <artifactId>lombok</artifactId>
+            <optional>true</optional>
+        </dependency>
         <dependency>
             <groupId>org.springframework.boot</groupId>
             <artifactId>spring-boot-starter-test</artifactId>