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>

shopdb/core/api/search.py

@@ -11,7 +11,7 @@ from sqlalchemy.orm import joinedload
 
 from shopdb.extensions import db
 from shopdb.core.models import (
-    Application, KnowledgeBase,
+    Application,
     Asset, AssetType, Communication, Vendor, Model
 )
 from shopdb.utils.responses import success_response
@@ -131,6 +131,8 @@ def _search_knowledgebase(query, search_term):
     """Search Knowledge Base by description and keywords."""
     results = []
     try:
+        _require_enabled('knowledgebase')
+        from plugins.knowledgebase.models import KnowledgeBase
         kb_articles = KnowledgeBase.query.filter(
             KnowledgeBase.isactive == True,
             db.or_(
@@ -153,6 +155,8 @@ def _search_knowledgebase(query, search_term):
                 'linkurl': kb.linkurl,
                 'relevance': relevance
             })
+    except ImportError:
+        pass  # knowledgebase plugin absent or disabled
     except Exception as e:
         logger.error(f"KnowledgeBase search failed: {e}")
     return results