feat(layout): Add Tiers to FacilitySection and Position

- Added 'tiers' to FacilitySection for multi-level racks
- Added 'tier' to FacilityPosition for Z-axis addressing
- Updated unique constraint for positions

backend/prisma/schema.prisma

@@ -587,6 +587,7 @@ model FacilitySection {
   height      Int
   rows        Int      // Grid rows
   columns     Int      // Grid columns
+  tiers       Int      @default(1) // Number of vertical tiers
   spacing     Int      @default(12) // inches between positions
   positions   FacilityPosition[]
   createdAt   DateTime @default(now())
@@ -601,11 +602,12 @@ model FacilityPosition {
   sectionId   String
   row         Int
   column      Int
+  tier        Int      @default(1) // Vertical level (1 = bottom)
   slot        Int      @default(1) // For multi-plant positions
   status      String   @default("EMPTY") // EMPTY, OCCUPIED, RESERVED, DAMAGED
   plant       FacilityPlant?
   
-  @@unique([sectionId, row, column, slot])
+  @@unique([sectionId, row, column, tier, slot])
   @@map("facility_positions")
 }