Extract Knowledge Base into a plugin (reference non-asset plugin)

First feature extracted from core into a plugin per "plugin is the product",
mirroring the notifications plugin. KB is a NON-asset plugin: it contributes a
model + blueprint + nav item but registers no AssetType.

- plugins/knowledgebase/: manifest.json (api_prefix /api/knowledgebase, no deps),
  models/ (KnowledgeBase, contract-pure imports via shopdb.api), api/ (the
  blueprint, same routes/prefix so the frontend is unchanged), plugin.py
  (get_blueprint + get_models + get_navigation_items).
- De-cored: removed shopdb/core/models/knowledgebase.py + api/knowledgebase.py,
  their __init__ exports, and 'knowledgebase' from CORE_BLUEPRINT_NAMES; dropped
  the hardcoded KB nav item from dashboard.py (now via the plugin nav hook).
- search.py and reports.py lazy-import KnowledgeBase from the plugin and degrade
  gracefully (search skips via _require_enabled when disabled; kb-popularity
  report returns 503 if the plugin is absent).
- Registered in instance/plugins.json (enabled).

The knowledgebase table stays in the core Alembic chain (bundled-plugin schema
folded into core, ADR-004); the model just maps it. KB was never in the
shopdb.api contract surface, so no __contract_version__ bump.

Pinned with characterization tests first (test_knowledgebase.py); they pass
unchanged against the plugin blueprint. 163 tests pass, naming green, app boots
7 bundled plugins, KB endpoint/nav/search verified live.

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

plugins/knowledgebase/plugin.py

@@ -0,0 +1,74 @@
+"""Knowledge Base plugin main class.
+
+Reference NON-asset plugin: contributes a model + blueprint + nav item but no
+AssetType (it is not an asset). Mirrors the notifications plugin.
+"""
+
+import json
+import logging
+from pathlib import Path
+from typing import List, Dict, Optional, Type
+
+from flask import Flask, Blueprint
+
+from shopdb.plugins.base import BasePlugin, PluginMeta
+
+from .models import KnowledgeBase
+from .api import knowledgebase_bp
+
+logger = logging.getLogger(__name__)
+
+
+class KnowledgeBasePlugin(BasePlugin):
+    """Knowledge Base plugin - articles linking to external resources."""
+
+    def __init__(self):
+        self._manifest = self._load_manifest()
+
+    def _load_manifest(self) -> Dict:
+        """Load plugin manifest from JSON file."""
+        manifest_path = Path(__file__).parent / 'manifest.json'
+        if manifest_path.exists():
+            with open(manifest_path, 'r') as f:
+                return json.load(f)
+        return {}
+
+    @property
+    def meta(self) -> PluginMeta:
+        """Return plugin metadata."""
+        return PluginMeta(
+            name=self._manifest.get('name', 'knowledgebase'),
+            version=self._manifest.get('version', '1.0.0'),
+            description=self._manifest.get(
+                'description',
+                'Knowledge Base articles linking to external resources'
+            ),
+            author=self._manifest.get('author', 'ShopDB Team'),
+            dependencies=self._manifest.get('dependencies', []),
+            core_version=self._manifest.get('core_version', '>=0.1.0,<1.0.0'),
+            api_prefix=self._manifest.get('api_prefix', '/api/knowledgebase'),
+        )
+
+    def get_blueprint(self) -> Optional[Blueprint]:
+        """Return Flask Blueprint with API routes."""
+        return knowledgebase_bp
+
+    def get_models(self) -> List[Type]:
+        """Return list of SQLAlchemy model classes."""
+        return [KnowledgeBase]
+
+    def init_app(self, app: Flask, db_instance) -> None:
+        """Initialize plugin with Flask app."""
+        logger.info(f"Knowledge Base plugin initialized (v{self.meta.version})")
+
+    def get_navigation_items(self) -> List[Dict]:
+        """Return navigation menu items."""
+        return [
+            {
+                'name': 'Knowledge Base',
+                'icon': 'book-open',
+                'route': '/knowledgebase',
+                'position': 35,
+                'section': 'information',
+            },
+        ]