fix: resolve ruff lint errors in alembic migration

- Replace typing.Union with X | Y syntax (UP007)
- Import Sequence from collections.abc instead of typing (UP035)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

alembic/versions/20260104_0000_initial_schema.py

@@ -5,16 +5,16 @@ Revises:
 Create Date: 2026-01-04 00:00:00.000000+00:00
 """
 
-from typing import Sequence, Union
+from collections.abc import Sequence
 
 import sqlalchemy as sa
 from alembic import op
 
 # revision identifiers, used by Alembic.
 revision: str = "001_initial"
-down_revision: Union[str, None] = None
-branch_labels: Union[str, Sequence[str], None] = None
-depends_on: Union[str, Sequence[str], None] = None
+down_revision: str | None = None
+branch_labels: str | Sequence[str] | None = None
+depends_on: str | Sequence[str] | None = None
 
 
 def upgrade() -> None: