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/plugin.py

@@ -9,9 +9,7 @@ from flask import Flask, Blueprint
 import click
 
 from shopdb.plugins.base import BasePlugin, PluginMeta
-from shopdb.extensions import db
-from shopdb.core.models.machine import MachineType
-from shopdb.core.models import AssetType
+from shopdb.api import db, AssetType
 
 from .models import Printer, PrinterType, ModelSupply
 from .api import printers_asset_bp
@@ -104,7 +102,6 @@ class PrintersPlugin(BasePlugin):
         with app.app_context():
             self._ensure_asset_type()
             self._ensure_printer_types()
-            self._ensure_legacy_machine_types()
             logger.info("Printers plugin installed")
 
     def _ensure_asset_type(self) -> None:
@@ -149,30 +146,6 @@ class PrintersPlugin(BasePlugin):
 
         db.session.commit()
 
-    def _ensure_legacy_machine_types(self) -> None:
-        """Ensure basic printer machine types exist (legacy architecture)."""
-        printertypes = [
-            ('Laser Printer', 'Printer', 'Standard laser printer'),
-            ('Inkjet Printer', 'Printer', 'Inkjet printer'),
-            ('Label Printer', 'Printer', 'Label/barcode printer'),
-            ('Multifunction Printer', 'Printer', 'MFP with scan/copy/fax'),
-            ('Plotter', 'Printer', 'Large format plotter'),
-        ]
-
-        for name, category, description in printertypes:
-            existing = MachineType.query.filter_by(machinetype=name).first()
-            if not existing:
-                mt = MachineType(
-                    machinetype=name,
-                    category=category,
-                    description=description,
-                    icon='printer'
-                )
-                db.session.add(mt)
-                logger.debug(f"Created machine type: {name}")
-
-        db.session.commit()
-
     def on_uninstall(self, app: Flask) -> None:
         """Called when plugin is uninstalled."""
         logger.info("Printers plugin uninstalled")