DB review safe-fix: naive-UTC timestamp defaults (drop db.func.now)

DB review found four DateTime columns defaulting to db.func.now() (MySQL
session-timezone wall clock) while the rest of the schema stores naive UTC, so
one schema mixed two clocks and to_dict() labelled the local values UTC with a
'Z' suffix. Switch application.dateadded, computers.installeddate,
knowledgebase.lastupdated (default + onupdate), and slides.uploadeddate to the
module-level naive-UTC _utcnow callable already used elsewhere (apitoken.py).
ORM-side default only - no column-type change, no data migration; affects
new/updated rows.

Targeted tests pass (154); naming + pyflakes green.

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

plugins/knowledgebase/models/knowledgebase.py

@@ -7,9 +7,16 @@ references the core applications table by name, which resolves at mapper config
 time without importing the core model.
 """
 
+from datetime import datetime, timezone
+
 from shopdb.api import db, BaseModel
 
 
+def _utcnow():
+    # naive UTC to match the other DB DateTime columns (stored without tzinfo)
+    return datetime.now(timezone.utc).replace(tzinfo=None)
+
+
 class KnowledgeBase(BaseModel):
     """Knowledge Base article linking to external resources."""
     __tablename__ = 'knowledgebase'
@@ -20,7 +27,7 @@ class KnowledgeBase(BaseModel):
     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())
+    lastupdated = db.Column(db.DateTime, default=_utcnow, onupdate=_utcnow)
 
     # Relationship to the core Application model (resolved by class name).
     application = db.relationship('Application', backref=db.backref('knowledgebase_articles', lazy='dynamic'))