DB backups before migrations (#420)

* make backups before migrations

* database tests

* postgres db backup with location string

* ignore version in pg_dump and throw warning

* install latest pg_dump

* install latest

* pg update?

* remove test from github

* skip for postgres on github actions

cashu/mint/migrations.py

@@ -497,3 +497,13 @@ async def m014_proofs_add_Y_column(db: Database):
 
         # recreate the balance views
         await create_balance_views(db, conn)
+
+
+async def m015_add_index_Y_to_proofs_used(db: Database):
+    # create index on proofs_used table for Y
+    async with db.connect() as conn:
+        await conn.execute(
+            "CREATE INDEX IF NOT EXISTS"
+            " proofs_used_Y_idx ON"
+            f" {table_with_schema(db, 'proofs_used')} (Y)"
+        )