feat(api): label layout management — first-class layouts, integration-scoped, multi-instance

[strausmann]

Implements ADR 0012.

Scope

• layouts SQL table with schema from ADR 0012
• Seed layouts on startup if missing (snipeit-asset-12mm, grocy-product-12mm, spoolman-spool-12mm, etc.)
• API endpoints:
  • GET /api/layouts (filter by integration, tape_mm, printer_model)
  • GET /api/layouts/{id}
  • GET /api/layouts/{id}/preview (rendered with synthetic data)
  • POST /api/layouts (create custom)
  • PUT /api/layouts/{id} (update — forbidden on is_seed=true)
  • POST /api/layouts/{id}/set-default (atomic)
  • DELETE /api/layouts/{id} (soft-delete)
• POST /api/print/{printer} accepts optional layout_id
• Webhook /api/webhook/grocy accepts optional layout_id
• Layout dropdown in the UI per integration tab
• LabelRenderer consumes Layout instead of hard-coded TemplateService

Acceptance

• Unique-default invariant per (integration, tape_mm)
• Seed layouts re-created on missing
• is_disabled vs deletion distinction works
• Preview cache invalidates on definition change

Out of scope

Visual editor (separate issue #17, post-MVP)