Add support for “secret” file URLs

Closes #47

migrations/versions/e2e816056589_.py

@@ -0,0 +1,26 @@
+"""add URL secret
+
+Revision ID: e2e816056589
+Revises: 0659d7b9eea8
+Create Date: 2022-12-01 02:16:15.976864
+
+"""
+
+# revision identifiers, used by Alembic.
+revision = 'e2e816056589'
+down_revision = '0659d7b9eea8'
+
+from alembic import op
+import sqlalchemy as sa
+
+
+def upgrade():
+    # ### commands auto generated by Alembic - please adjust! ###
+    op.add_column('file', sa.Column('secret', sa.String(), nullable=True))
+    # ### end Alembic commands ###
+
+
+def downgrade():
+    # ### commands auto generated by Alembic - please adjust! ###
+    op.drop_column('file', 'secret')
+    # ### end Alembic commands ###