Correct applied_at type annotation to str

_AppliedMigration.applied_at was annotated datetime.datetime but
the value is the TEXT timestamp read straight from the
_sqlite_migrations table - always a string, matching the format
written by both this module and the older sqlite-migrate package.
Added a test pinning the runtime type.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UnLnhsH25Nnv7LHhekUfPd

sqlite_utils/migrations.py

@@ -19,7 +19,9 @@ class Migrations:
     @dataclass
     class _AppliedMigration:
         name: str
-        applied_at: datetime.datetime
+        # A string timestamp such as "2026-07-04 12:00:00.000000+00:00" -
+        # stored as TEXT in the _sqlite_migrations table
+        applied_at: str
 
     def __init__(self, name: str):
         """

tests/test_migrations.py

@@ -80,6 +80,18 @@ def test_order_does_not_matter(migrations, migrations_not_ordered_alphabetically
     assert db1.schema == db2.schema
 
 
+def test_applied_at_is_a_string(migrations):
+    db = sqlite_utils.Database(memory=True)
+    migrations.apply(db)
+    applied = migrations.applied(db)
+    assert len(applied) == 2
+    for migration in applied:
+        # applied_at is the TEXT timestamp straight from the
+        # _sqlite_migrations table, e.g. "2026-07-04 12:00:00.000000+00:00"
+        assert isinstance(migration.applied_at, str)
+        assert migration.applied_at.endswith("+00:00")
+
+
 def test_failing_migration_rolls_back(migrations):
     @migrations()
     def m003(db):