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/models/knowledgebase.py

@@ -0,0 +1,33 @@
+"""Knowledge Base model.
+
+Non-asset plugin model. The `knowledgebase` table lives in the core Alembic
+chain (bundled-plugin schema is folded into core, ADR-004); this class just
+maps it and is registered via the plugin's get_models hook. The appid FK
+references the core applications table by name, which resolves at mapper config
+time without importing the core model.
+"""
+
+from shopdb.api import db, BaseModel
+
+
+class KnowledgeBase(BaseModel):
+    """Knowledge Base article linking to external resources."""
+    __tablename__ = 'knowledgebase'
+
+    linkid = db.Column(db.Integer, primary_key=True)
+    appid = db.Column(db.Integer, db.ForeignKey('applications.appid'))
+    shortdescription = db.Column(db.String(500), nullable=False)
+    linkurl = db.Column(db.String(2000))
+    keywords = db.Column(db.String(500))
+    clicks = db.Column(db.Integer, default=0)
+    lastupdated = db.Column(db.DateTime, default=db.func.now(), onupdate=db.func.now())
+
+    # Relationship to the core Application model (resolved by class name).
+    application = db.relationship('Application', backref=db.backref('knowledgebase_articles', lazy='dynamic'))
+
+    def __repr__(self):
+        return f"<KnowledgeBase {self.linkid}: {self.shortdescription[:50] if self.shortdescription else 'No desc'}>"
+
+    def increment_clicks(self):
+        """Increment click counter."""
+        self.clicks = (self.clicks or 0) + 1