feat(api): REST endpoints + OpenAPI

[strausmann]

Goal

Implement all REST endpoints from the spec (printers, jobs, lookup, webhooks, QR-scan landing pages).

Endpoints

• GET /api/printers — list with cached status
• GET /api/printers/{id}/status — force fresh ESC i S read
• GET /api/printers/{id}/tape — current tape spec
• GET /api/printers/{id}/queue — list queued+paused+printing
• POST /api/printers/{id}/{pause,resume,queue/clear}
• GET /api/templates
• GET /api/lookup/{app}/{id}
• POST /api/print/{printer_id} → 202 + JobId
• GET /api/jobs, /api/jobs/{id}
• POST /api/jobs/{id}/{pause,resume,cancel,retry,priority}
• POST /api/webhook/{spoolman,grocy} (API-Key auth)
• GET /loc/{id}, /asset/{id}, /spool/{id}, /product/{id} — QR landing pages
• GET /healthz (open)
• GET /api/events — SSE stream

Tasks

• Pydantic schemas for all request/response types
• Dependency-injection setup (PrinterService, AppLookupService, ...)
• OpenAPI tags + descriptions for auto-generated docs
• Webhook API-key middleware
• HTTP error mapping (Brother errors → meaningful 4xx/5xx)