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/exceptions.py

@@ -0,0 +1,70 @@
+"""Custom exceptions for ShopDB."""
+
+
+class ShopDBException(Exception):
+    """Base exception for ShopDB."""
+
+    def __init__(self, message: str, code: str = None, details: dict = None):
+        super().__init__(message)
+        self.message = message
+        self.code = code or 'SHOPDB_ERROR'
+        self.details = details or {}
+
+
+class ValidationError(ShopDBException):
+    """Validation error."""
+
+    def __init__(self, message: str, details: dict = None):
+        super().__init__(message, 'VALIDATION_ERROR', details)
+
+
+class NotFoundError(ShopDBException):
+    """Resource not found error."""
+
+    def __init__(self, resource: str, identifier):
+        message = f"{resource} with ID {identifier} not found"
+        super().__init__(message, 'NOT_FOUND', {'resource': resource, 'id': identifier})
+
+
+class AuthenticationError(ShopDBException):
+    """Authentication error."""
+
+    def __init__(self, message: str = "Authentication required"):
+        super().__init__(message, 'UNAUTHORIZED')
+
+
+class AuthorizationError(ShopDBException):
+    """Authorization error."""
+
+    def __init__(self, message: str = "Permission denied"):
+        super().__init__(message, 'FORBIDDEN')
+
+
+class ConflictError(ShopDBException):
+    """Conflict error (e.g., duplicate entry)."""
+
+    def __init__(self, message: str, details: dict = None):
+        super().__init__(message, 'CONFLICT', details)
+
+
+class PluginError(ShopDBException):
+    """Plugin-related error."""
+
+    def __init__(self, message: str, plugin_name: str = None):
+        super().__init__(message, 'PLUGIN_ERROR', {'plugin': plugin_name})
+
+
+class PluginNotFoundError(PluginError):
+    """A plugin directory or manifest is missing."""
+
+
+class PluginContractError(PluginError):
+    """A plugin violates the BasePlugin contract (no subclass, missing hooks)."""
+
+
+class PluginVersionError(PluginError):
+    """A plugin's core_version range does not include the framework's __contract_version__."""
+
+
+class PluginDependencyError(PluginError):
+    """A plugin declares a dependency on another plugin that is missing or not enabled."""