feat: Story 024 — Seating Plans

[iagocavalcante]

Summary

Implements seating plans for seated events (theater, conferences, galas). Closes #24.

• Seating context with full CRUD for plans, sections, seats, and reservations
• JSON layout upload — organizers upload a venue layout that auto-generates sections and seats
• Seat reservations — hold during checkout, confirm on order, release on cancellation
• Concurrency-safe — partial unique index on [seat_id, event_id] prevents double-booking at DB level
• Section-to-item mapping — sections link to ticket types (items/variations)
• Reusable plans — plans belong to orgs and can be assigned to multiple events
• Manual assignment — organizers can assign/reassign seats for existing orders
• Admin LiveViews — index (list + upload) and show (details + section mapping + event assignment)

New schemas

• seating_plans — org-level venue layouts
• seating_sections — zones within a plan (mapped to items)
• seats — individual seats with row/number
• seat_reservations — per-event seat holds and confirmations

Test coverage

• 47 context tests (CRUD, holds, confirms, releases, concurrency, edge cases)
• 18 LiveView tests (index, show, upload, mapping, assignment)

Test plan

• 1565 tests pass, 0 failures
• Manual: upload JSON layout, verify sections/seats created
• Manual: assign plan to event, verify seat availability
• Manual: hold → confirm → release flow

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