feat: SQLModel persistence layer

[strausmann]

Goal

Implement SQLModel models + migrations for printers, templates, presets, jobs, printer_state, printer_status_cache.

Schema (from spec)

See full schema in Brother PT raster spec — relevant subset:

• printers
• templates (seed-loaded)
• presets
• jobs (UUID id, state machine)
• printer_state (worker active/paused)
• printer_status_cache (last ESC i S block)

Tasks

• SQLModel classes
• Async session/engine setup with aiosqlite
• Migration tool (Alembic? or hand-written for v1)
• Seed loader for 6 default templates
• On startup: any queued/printing job → failed_restart (retry candidate)