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/printers/api/asset_routes.py

@@ -5,15 +5,7 @@ import logging
 from flask import Blueprint, request
 from flask_jwt_extended import jwt_required
 
-from shopdb.extensions import db, cache
-from shopdb.core.models import Asset, AssetType, Vendor, Model, Communication, CommunicationType
-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, cache, Asset, AssetType, Vendor, Model, Communication, CommunicationType, success_response, error_response, paginated_response, ErrorCodes, get_pagination_params, paginate_query
 
 from ..models import Printer, PrinterType, ModelSupply
 from ..models.model_supply import SUPPLY_TYPES, SUPPLY_COLORS, CAPACITY_TIERS
@@ -565,7 +557,7 @@ def _get_low_supplies_data():
             # location name for the report row
             location_name = None
             if asset.locationid:
-                from shopdb.core.models import Location
+                from shopdb.api import Location
                 loc = Location.query.get(asset.locationid)
                 if loc:
                     location_name = loc.locationname