Initial import: shopdb-flask multi-site asset platform

Flask + Vue rewrite of the classic-ASP shopdb, built as a plugin
framework so sister GE Aerospace sites can adopt it. Product version
0.7.0-pre-0.8.0, plugin contract 0.11.0, 966 backend tests.

Key entry points:
- docs/INSTALL-WINDOWS-IIS.md - tested Windows + IIS install runbook
  (own-site and subpath deployment methods)
- docs/PILOT-DEPLOY.md - production pilot: stand up + import + verify
- docs/IMPORT-API.md - schema-agnostic legacy data import contract
- scripts/site_imports/wjf/ - West Jefferson reference import loader
- docs/adr/ - twelve accepted architecture decision records

shopdb/core/models/base.py

@@ -0,0 +1,71 @@
+"""Base model class with common fields."""
+
+from datetime import datetime, timezone
+from shopdb.extensions import db
+
+
+def _utcnow():
+    # naive UTC for DB columns (stored without tzinfo)
+    return datetime.now(timezone.utc).replace(tzinfo=None)
+
+
+class BaseModel(db.Model):
+    """
+    Abstract base model with common fields.
+    All models should inherit from this.
+    """
+    __abstract__ = True
+
+    createddate = db.Column(
+        db.DateTime,
+        default=_utcnow,
+        nullable=False
+    )
+    modifieddate = db.Column(
+        db.DateTime,
+        default=_utcnow,
+        onupdate=_utcnow,
+        nullable=False
+    )
+    isactive = db.Column(db.Boolean, default=True, nullable=False)
+
+    def to_dict(self):
+        """Convert model to dictionary."""
+        result = {}
+        for c in self.__table__.columns:
+            value = getattr(self, c.name)
+            if isinstance(value, datetime):
+                value = value.isoformat() + 'Z'
+            result[c.name] = value
+        return result
+
+    def update(self, **kwargs):
+        """Update model attributes."""
+        for key, value in kwargs.items():
+            if hasattr(self, key):
+                setattr(self, key, value)
+
+    @classmethod
+    def get_active(cls):
+        """Return query for active records only."""
+        return cls.query.filter_by(isactive=True)
+
+
+class SoftDeleteMixin:
+    """Mixin for soft delete functionality."""
+
+    deleteddate = db.Column(db.DateTime, nullable=True)
+    deletedby = db.Column(db.String(100), nullable=True)
+
+    def soft_delete(self, deleted_by: str = None):
+        """Mark record as deleted."""
+        self.isactive = False
+        self.deleteddate = datetime.now(timezone.utc).replace(tzinfo=None)
+        self.deletedby = deleted_by
+
+
+class AuditMixin:
+    """Mixin for audit fields."""
+
+    createdby = db.Column(db.String(100), nullable=True)
+    modifiedby = db.Column(db.String(100), nullable=True)