feat: store generated invoices per mint

lnbits/extensions/cashu/migrations.py

@@ -60,4 +60,19 @@ async def m001_initial(db):
             cashu_id TEXT NOT NULL
         );
     """
+    )
+
+    await db.execute(
+        """
+            CREATE TABLE IF NOT EXISTS cashu.invoices (
+                cashu_id TEXT NOT NULL,
+                amount INTEGER NOT NULL,
+                pr TEXT NOT NULL,
+                hash TEXT NOT NULL,
+                issued BOOL NOT NULL,
+
+                UNIQUE (hash)
+
+            );
+        """
     )