refactor: improve database migrations

lnbits/extensions/paywall/migrations.py

@@ -0,0 +1,22 @@
+from lnbits.db import open_ext_db
+
+
+def m001_initial(db):
+    """
+    Initial paywalls table.
+    """
+    db.execute("""
+        CREATE TABLE IF NOT EXISTS paywalls (
+            id TEXT PRIMARY KEY,
+            wallet TEXT NOT NULL,
+            url TEXT NOT NULL,
+            memo TEXT NOT NULL,
+            amount INTEGER NOT NULL,
+            time TIMESTAMP NOT NULL DEFAULT (strftime('%s', 'now'))
+        );
+    """)
+
+
+def migrate():
+    with open_ext_db("tpos") as db:
+        m001_initial(db)