Retire the legacy Machine model (ADR-001 cutover)

The asset/computer model is now the single source of truth. Remove the Machine
instance layer end to end:

- Delete models Machine, MachineStatus, PCType, MachineRelationship,
  InstalledApp, PrinterData; keep MachineType (models.machinetypeid still
  references it).
- Delete the /api/machines, /api/statuses, /api/pctypes blueprints and the
  legacy /api/printers/legacy (PrinterData) blueprint.
- Drop the deprecated communications.machineid column and its FK.
- Migration 7c01 drops tables machines, machinestatuses, pctypes,
  machinerelationships, installedapps, printerdata (idempotent).
- Fix remaining readers (applications install counts) to ComputerInstalledApp.
- Frontend: remove dead machinesApi/statusesApi/pctypesApi wrappers; repoint
  the PC Types settings page at computer types.

143 tests pass; all asset/computer/dashboard/report/collector endpoints 200.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

plugins/printers/api/__init__.py

@@ -1,9 +1,7 @@
 """Printers plugin API."""
 
-from .routes import printers_bp  # Legacy Machine-based API
-from .asset_routes import printers_asset_bp  # New Asset-based API
+from .asset_routes import printers_asset_bp  # Asset-based API
 
 __all__ = [
-    'printers_bp',         # Legacy
-    'printers_asset_bp',   # New
+    'printers_asset_bp',
 ]