FP-19: Stairs

[steilerDev]

As a homeowner, I want to place and visualize stairs on floor plans so that I can document vertical circulation and navigate between floor levels.

Parent Epic: #752 (EPIC-16: Floor Plans & Utility Tracking)
Priority: Should Have
Phase: 5 — Multi-Floor & Special Structures

Acceptance Criteria

• 1. GET /api/floor-plans/stairs with query param floorId returns all stairs that touch a given floor
• 2. POST /api/floor-plans/stairs creates a stair with type (straight/l_shaped/u_shaped/spiral), from_floor_id, to_floor_id, position (polygon coordinates), step_count, and optional label
• 3. PATCH /api/floor-plans/stairs/:id updates stair properties
• 4. DELETE /api/floor-plans/stairs/:id deletes a stair
• 5. Stairs are rendered on the floor plan canvas as hatched rectangles/polygons at their stored position
• 6. Each stair shows a directional arrow indicating up/down and the step count as a text label
• 7. The stair type affects the rendering shape: straight (rectangle), l_shaped (L polygon), u_shaped (U polygon), spiral (circle with spiral lines)
• 8. Clicking a stair on the floor plan navigates to the connected floor's plan view
• 9. A stair appears on both its from_floor and to_floor plan views
• 10. Create/edit dialog includes type selector, from/to floor dropdowns (limited to floors in the same building), step count, and position adjustment
• 11. Validation: from_floor_id and to_floor_id must be different floors in the same building; step_count must be a positive integer

Notes

• Dependencies: FP-6 (FP-6: Floor Plan Canvas (Read-Only) #758) — floor plan canvas; FP-4 (FP-4: Floors CRUD & Building Detail #756) — floors CRUD
• Stairwell walls are regular walls drawn around the stair opening — they have full layer and utility support
• The stair polygon defines the floor opening/void, not the walls around it