Enforce plugin contract purity: single import surface via shopdb.api

Plugins were reaching into internal core paths (shopdb.core.models.*,
shopdb.extensions, shopdb.utils.*), coupling them to core's file layout and
violating the ADR-001 contract. Consolidate onto one versioned surface.

- shopdb.api: expand from 2 helpers to the full plugin import surface -
  db, cache; BaseModel, AuditMixin; core models (Asset, AssetType,
  AssetStatus, Vendor, Model, Communication, CommunicationType, Location,
  Setting, AuditLog, Application, AppVersion, OperatingSystem); response +
  pagination helpers; employee_connection. Documented in PLUGIN-HOOKS.md.
- Migrate all 22 plugin source files to import only from shopdb.api (plus
  shopdb.plugins.base for the ABC).
- Drop the printers plugin's legacy MachineType dependency: remove
  _ensure_legacy_machine_types and the seed_supplies machinetypeid lookup
  (Model.machinetypeid is nullable; printers carry type via PrinterType).
- Guard test test_plugins_only_import_contract_surface scans plugin source
  and fails on any core import outside shopdb.api / shopdb.plugins.base.
- Scaffold templates updated so generated plugins are contract-pure.
- Bump __contract_version__ 0.2.0 -> 0.3.0 (additive surface expansion;
  manifests pin <1.0.0 so they still satisfy).

145 tests pass, naming/style green, app factory boots all 6 plugins.

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

plugins/usb/api/routes.py

@@ -4,15 +4,7 @@ from flask import Blueprint, request
 from flask_jwt_extended import jwt_required, get_jwt_identity
 from datetime import datetime
 
-from shopdb.extensions import db
-from shopdb.core.models import AuditLog
-from shopdb.utils.responses import (
-    success_response,
-    error_response,
-    paginated_response,
-    ErrorCodes
-)
-from shopdb.utils.pagination import get_pagination_params, paginate_query
+from shopdb.api import db, AuditLog, success_response, error_response, paginated_response, ErrorCodes, get_pagination_params, paginate_query
 
 from ..models import USBDevice, USBDeviceType, USBCheckout