Added a new backend settings system

This can be used for saving runtime-configurable settings in the database.

spoolman/settings.py

@@ -0,0 +1,64 @@
+"""Settings that can be changed by the user.
+
+All settings are JSON encoded and stored in the database.
+"""
+
+import json
+from dataclasses import dataclass
+from enum import Enum
+
+
+class SettingType(Enum):
+    """The type of a setting."""
+
+    BOOLEAN = "boolean"
+    NUMBER = "number"
+    STRING = "string"
+    ARRAY = "array"
+    OBJECT = "object"
+
+
+@dataclass
+class SettingDefinition:
+    """A setting that can be changed by the user."""
+
+    key: str
+    type: SettingType
+    default: str
+
+    def validate_type(self, value: str) -> None:  # noqa: C901
+        """Validate that the value has the correct type."""
+        obj = json.loads(value)
+        if self.type == SettingType.BOOLEAN:
+            if not isinstance(obj, bool):
+                raise ValueError(f"Setting {self.key} must be a boolean.")
+        elif self.type == SettingType.NUMBER:
+            if not isinstance(obj, (int, float)):
+                raise ValueError(f"Setting {self.key} must be a number.")
+        elif self.type == SettingType.STRING:
+            if not isinstance(obj, str):
+                raise ValueError(f"Setting {self.key} must be a string.")
+        elif self.type == SettingType.ARRAY:
+            if not isinstance(obj, list):
+                raise ValueError(f"Setting {self.key} must be an array.")
+        elif self.type == SettingType.OBJECT:  # noqa: SIM102
+            if not isinstance(obj, dict):
+                raise ValueError(f"Setting {self.key} must be an object.")
+
+
+SETTINGS: dict[str, SettingDefinition] = {}
+
+
+def register_setting(key: str, typ: SettingType, default: str) -> None:
+    """Register a setting."""
+    SETTINGS[key] = SettingDefinition(key, typ, default)
+
+
+def parse_setting(key: str) -> SettingDefinition:
+    """Parse a setting key."""
+    if key not in SETTINGS:
+        raise ValueError(f"Setting {key} does not exist.")
+    return SETTINGS[key]
+
+
+register_setting("currency", SettingType.STRING, json.dumps("EUR"))