Added a new backend settings system

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

migrations/README.md

@@ -1,7 +1,18 @@
 # Migrations
 
-Creating a new version:
+Migrations are used to create and update the database schema. They are run automatically every time Spoolman starts.
+
+To create a new migration, edit the tables as desired in `spoolman/database/models.py`, then start the Spoolman server to update your local sqlite database.
+
 ```bash
 pdm run python -m spoolman.main
+```
+
+Stop the server once it's up.
+
+Then, let Alembic automatically create a new migration file:
+```bash
 pdm run alembic revision -m "some title" --autogenerate
 ```
+
+Go into the created migration and make sure it looks good, that the column changes etc are as desired. Format it with Black and Ruff. Commit.

migrations/versions/2024_01_03_1346-ccbb17aeda7c_added_settings_table.py

@@ -0,0 +1,32 @@
+"""Added Settings table.
+
+Revision ID: ccbb17aeda7c
+Revises: 92793c8a937c
+Create Date: 2024-01-03 13:46:41.362341
+"""
+import sqlalchemy as sa
+from alembic import op
+
+# revision identifiers, used by Alembic.
+revision = "ccbb17aeda7c"
+down_revision = "92793c8a937c"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    """Perform the upgrade."""
+    op.create_table(
+        "setting",
+        sa.Column("key", sa.String(length=64), nullable=False),
+        sa.Column("value", sa.Text(), nullable=False),
+        sa.Column("last_updated", sa.DateTime(), nullable=False),
+        sa.PrimaryKeyConstraint("key"),
+    )
+    op.create_index(op.f("ix_setting_key"), "setting", ["key"], unique=False)
+
+
+def downgrade() -> None:
+    """Perform the downgrade."""
+    op.drop_index(op.f("ix_setting_key"), table_name="setting")
+    op.drop_table("setting")