Fix ALTER TABLE migration for created_at column

SQLite ALTER TABLE ADD COLUMN does not support DEFAULT expressions,
only literal values. Use DEFAULT '' then UPDATE existing rows.

src/infrastructure/db.rs

@@ -47,7 +47,11 @@ impl Db {
         )
         .ok();
         conn.execute_batch(
-            "ALTER TABLE tasks ADD COLUMN created_at TEXT NOT NULL DEFAULT (strftime('%Y-%m-%d %H:%M:%S', 'now'));",
+            "ALTER TABLE tasks ADD COLUMN created_at TEXT NOT NULL DEFAULT '';",
+        )
+        .ok();
+        conn.execute_batch(
+            "UPDATE tasks SET created_at = updated_at WHERE created_at = '';",
         )
         .ok();
         Ok(Self { conn: Mutex::new(conn) })