Chore: Exclude alembic/ from Ruff linting

Alembic scaffolds migration files from its own template which uses
pre-3.10 conventions (Union[X, Y], typing.Sequence, etc). Excluding
avoids noise on every new migration without affecting app code coverage.

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

api/pyproject.toml

@@ -30,6 +30,7 @@ dev = [
 [tool.ruff]
 line-length = 100
 target-version = "py312"
+exclude = ["alembic/"]
 
 [tool.ruff.lint]
 select = ["E", "F", "I", "UP", "B", "SIM"]