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>

shopdb/core/models/application.py

@@ -1,7 +1,14 @@
 """Application tracking models."""
 
+from datetime import datetime, timezone
+
 from shopdb.extensions import db
 from .base import BaseModel
+
+
+def _utcnow():
+    # naive UTC to match the other DB DateTime columns (stored without tzinfo)
+    return datetime.now(timezone.utc).replace(tzinfo=None)
 # SupportTeam / SupportTeamContact live in supportteam.py; imported by the
 # models package so the Application.supportteam relationship resolves.
 
@@ -61,7 +68,7 @@ class AppVersion(db.Model):
     version = db.Column(db.String(50), nullable=False)
     releasedate = db.Column(db.Date)
     notes = db.Column(db.String(255))
-    dateadded = db.Column(db.DateTime, default=db.func.now())
+    dateadded = db.Column(db.DateTime, default=_utcnow)
     isactive = db.Column(db.Boolean, default=True)
 
     # Relationships