feat(api): add REST controllers, services, entities and 15-min timeline grid

src/main/java/com/caspos/repository/GuestRepository.java

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+package com.caspos.repository;
+
+import com.caspos.domain.Guest;
+import org.springframework.data.jpa.repository.JpaRepository;
+import org.springframework.stereotype.Repository;
+
+import java.util.UUID;
+
+@Repository
+public interface GuestRepository extends JpaRepository<Guest, UUID> {
+}

src/main/java/com/caspos/repository/ReservationRepository.java

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+package com.caspos.repository;
+
+import com.caspos.domain.Reservation;
+import org.springframework.data.jpa.repository.JpaRepository;
+import org.springframework.data.jpa.repository.Query;
+import org.springframework.data.repository.query.Param;
+import org.springframework.stereotype.Repository;
+
+import java.time.LocalDate;
+import java.time.OffsetDateTime;
+import java.util.List;
+import java.util.UUID;
+
+@Repository
+public interface ReservationRepository extends JpaRepository<Reservation, UUID> {
+
+    @Query("""
+        SELECT r FROM Reservation r
+        WHERE CAST(r.startTime AS date) = :date
+        ORDER BY r.startTime ASC
+    """)
+    List<Reservation> findByDate(@Param("date") LocalDate date);
+
+    @Query("""
+        SELECT r FROM Reservation r
+        WHERE r.startTime < :end AND r.endTime > :start
+        AND r.status NOT IN ('CANCELLED', 'NO_SHOW')
+    """)
+    List<Reservation> findOverlapping(@Param("start") OffsetDateTime start, @Param("end") OffsetDateTime end);
+}

src/main/java/com/caspos/repository/TableRepository.java

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+package com.caspos.repository;
+
+import com.caspos.domain.RestaurantTable;
+import org.springframework.data.jpa.repository.JpaRepository;
+import org.springframework.stereotype.Repository;
+
+import java.util.List;
+import java.util.UUID;
+
+@Repository
+public interface TableRepository extends JpaRepository<RestaurantTable, UUID> {
+    List<RestaurantTable> findByDiningAreaId(UUID diningAreaId);
+    List<RestaurantTable> findAllByOrderByDiningAreaDisplayOrderAscTableNumberAsc();
+}