Plugin system: fix PluginMeta core_version default + add /api/plugins

- PluginMeta.core_version defaulted to >=1.0.0, which would reject the current
  0.2.0 framework for any plugin relying on the default. Set to >=0.2.0,<1.0.0.
- Add GET /api/plugins introspection: lists loaded plugins (name, version,
  core_version, api_prefix, dependencies) + the framework contract version.

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

shopdb/core/api/plugins.py

@@ -0,0 +1,37 @@
+"""Plugin introspection API - what plugins are loaded and their contracts."""
+
+from flask import Blueprint, current_app
+from flask_jwt_extended import jwt_required
+
+from shopdb.utils.responses import success_response
+
+plugins_bp = Blueprint('plugins', __name__)
+
+
+@plugins_bp.route('', methods=['GET'])
+@jwt_required(optional=True)
+def list_plugins():
+    """List loaded plugins with their manifest metadata + the framework
+    contract version they were checked against."""
+    from shopdb import __contract_version__
+    pm = current_app.extensions.get('plugin_manager')
+    plugins = []
+    if pm:
+        for name, plugin in pm.get_all_plugins().items():
+            meta = plugin.meta
+            plugins.append({
+                'name': meta.name,
+                'version': meta.version,
+                'description': meta.description,
+                'author': meta.author,
+                'core_version': meta.core_version,
+                'api_prefix': meta.api_prefix,
+                'dependencies': meta.dependencies,
+            })
+    plugins.sort(key=lambda p: p['name'])
+
+    return success_response({
+        'contract_version': __contract_version__,
+        'count': len(plugins),
+        'plugins': plugins,
+    })