Phase 4: plugin scaffolding (flask plugin new) with canary tests

Lowers the barrier for sister sites to build their own plugins.
Generated output satisfies the framework contract out of the box.

CLI command (shopdb/plugins/cli.py):
- `flask plugin new <name> --description "..."` generates a plugin
  skeleton under plugins/<name>/. Validates the name against
  CONTRIBUTING.md rules (lowercase letters/digits only, no
  underscores or hyphens, not in the reserved list) and refuses to
  overwrite existing plugins unless --overwrite is passed.
- Output prints the next steps (install, migrate, test).

Scaffolder (shopdb/plugins/scaffolder.py):
- validate_name: enforces the naming rules
- pascal_case: lowercase-to-PascalCase for class names
- scaffold_plugin: copies templates with string.Template
  substitution. Three placeholders: $name, $Name, $description.
  Files with `model.py` in the path get renamed to <name>.py.

Templates (shopdb/plugins/templates/):
- manifest.json.tmpl: name, version 0.1.0, description, core_version
  range >=0.1.0,<1.0.0 (broad enough to survive minor framework bumps)
- plugin.py.tmpl: <Name>Plugin class extending BasePlugin with all
  required hooks implemented (meta from manifest, get_blueprint
  returning the bp, get_models returning the example model). Includes
  on_install hook that seeds the AssetType row.
- models/__init__.py.tmpl + models/model.py.tmpl: Asset extension
  table keyed by assetid with one example field. TODO comment marks
  it as a placeholder.
- api/__init__.py.tmpl + api/routes.py.tmpl: Blueprint with list and
  detail endpoints using the framework's pagination + response helpers.
- schemas/__init__.py.tmpl: marshmallow schema stub.
- tests/__init__.py.tmpl + tests/test_plugin.py.tmpl: smoke tests
  asserting plugin loads, get_blueprint returns Blueprint, get_models
  returns at least one model.
- README.md.tmpl: one-pager for plugin authors with common edits and
  next-step references.

Canary tests (tests/test_plugin_scaffold.py):
- 14 tests asserting the scaffold output passes contract checks.
- Validates name rules (lowercase, reserved, hyphens, digits, etc.)
- Verifies all expected files generated, manifest fields present.
- Loads the generated plugin via PluginLoader (spec_from_file_location
  bypasses the real `plugins` package shadowing).
- Asserts subclasses BasePlugin, get_blueprint returns Blueprint,
  get_models returns model with __tablename__.
- Module-scoped fixture; cleans up sys.modules + SQLAlchemy metadata
  on teardown to avoid cross-test contamination.

Quickstart docs (docs/PLUGIN-QUICKSTART.md):
- 30-minute walkthrough: scaffold -> edit model -> add routes ->
  install -> verify -> add hooks. Cross-links to PLUGIN-HOOKS.md and
  the ADRs. Includes common-errors table.

Test count: 87 -> 101 passing.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

shopdb/plugins/templates/models/__init__.py.tmpl

@@ -0,0 +1,5 @@
+"""$Name plugin models."""
+
+from .$name import $Name
+
+__all__ = ['$Name']