style: apply ruff's automatic fixes and formatter

Mechanical only, and separated from the judgment calls that follow so the
reviewable changes are not buried in a 98-file whitespace diff.

227 automatic fixes: 60 blank lines carrying whitespace, 60 unsorted import
blocks, 34 Optional[X] to X | None, 28 unused imports, 16 deprecated typing
imports, 12 datetime.timezone.utc to datetime.UTC, and assorted smaller
modernisations. Then `ruff format` over src and tests: 98 files reformatted,
35 already conforming.

No file among the unused-import findings defines __all__ or is an __init__.py,
so nothing here removes a re-export.

`make test`: 658 passed, unchanged from HEAD.

Two things observed while verifying, neither addressed here:

`pytest tests/` cannot collect — tests/e2e/test_orchestration_e2e.py uses an
`e2e` marker that is not registered, and the config is strict about markers.
This fails identically at HEAD, so it predates this change; `make test` passes
because it ignores tests/e2e, tests/integration and tests/contracts.

test_tatlock_tool_call_logging_calculator is flaky. It failed once in a full run
with these changes and passed on the next, passes in isolation with them, and
fails in isolation at HEAD. It is order- or timing-dependent, not a regression
from this commit — established by running the full suite both ways rather than
by reasoning about which change could have caused it.

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

tests/core/test_router.py

@@ -1,6 +1,7 @@
 """
 Tests for core router (health check and root endpoints).
 """
+
 import pytest
 from fastapi.testclient import TestClient
 
@@ -11,7 +12,7 @@ from src.core.config import config
 def test_health_check(client: TestClient) -> None:
     """Test health check endpoint returns healthy status."""
     response = client.get("/health")
-    
+
     assert response.status_code == 200
     data = response.json()
     assert data["status"] == "healthy"
@@ -22,7 +23,7 @@ def test_health_check(client: TestClient) -> None:
 def test_root_endpoint(client: TestClient) -> None:
     """Test root endpoint returns API information."""
     response = client.get("/")
-    
+
     assert response.status_code == 200
     data = response.json()
     assert data["name"] == config.APP_NAME
@@ -42,7 +43,7 @@ def test_openapi_schema(client: TestClient) -> None:
     """Test that OpenAPI schema is accessible."""
     response = client.get("/openapi.json")
     assert response.status_code == 200
-    
+
     schema = response.json()
     assert schema["info"]["title"] == config.APP_NAME
     assert schema["info"]["version"] == config.APP_VERSION