Fix: SQLite migration syntax error in m006

[PatMulligan]

Ran into this issue on my lnbits 1.4 on NixOS using the flake with SQLite

• Fix m006_add_extra_fields migration that fails on SQLite with syntax error
• Split multi-column ALTER TABLE into separate statements (SQLite doesn't support adding multiple columns in one statement)

LNBits logs snippet

running migration events.6
2025-12-31 16:04:24.76 | WARNING | (sqlite3.OperationalError) near ",": syntax error
[SQL:
        ALTER TABLE events.events
        ADD COLUMN canceled BOOLEAN NOT NULL DEFAULT FALSE,
        ADD COLUMN extra TEXT;
        ]
(Background on this error at: https://sqlalche.me/e/14/e3q8)