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/computers/models/computer.py

@@ -1,8 +1,15 @@
 """Computer plugin models."""
 
+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 ComputerType(BaseModel):
     """
     Computer type classification.
@@ -170,7 +177,7 @@ class ComputerInstalledApp(db.Model):
     # Raw version string from automated collection (when no curated AppVersion)
     installedversion = db.Column(db.String(100), nullable=True)
     isactive = db.Column(db.Boolean, default=True, nullable=False)
-    installeddate = db.Column(db.DateTime, default=db.func.now())
+    installeddate = db.Column(db.DateTime, default=_utcnow)
 
     # Relationships
     computer = db.relationship('Computer', back_populates='installedapps')