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>

src/main.py

@@ -9,8 +9,9 @@ Main responsibilities:
 - Router registration
 - Lifecycle management
 """
+
+from collections.abc import AsyncGenerator
 from contextlib import asynccontextmanager
-from typing import AsyncGenerator
 
 from fastapi import FastAPI, Request, status
 from fastapi.exceptions import RequestValidationError
@@ -64,7 +65,7 @@ async def lifespan(app: FastAPI) -> AsyncGenerator[None, None]:
 def create_application() -> FastAPI:
     """
     Application factory.
-    
+
     Creates and configures the FastAPI application.
     Following best practice of using factory pattern.
     """
@@ -75,7 +76,7 @@ def create_application() -> FastAPI:
         lifespan=lifespan,
         debug=config.DEBUG,
     )
-    
+
     # Add middleware
     application.add_middleware(
         CORSMiddleware,
@@ -84,10 +85,10 @@ def create_application() -> FastAPI:
         allow_methods=config.CORS_ALLOW_METHODS,
         allow_headers=config.CORS_ALLOW_HEADERS,
     )
-    
+
     # Register exception handlers
     register_exception_handlers(application)
-    
+
     # Include routers
     application.include_router(core_router)  # Health and root endpoints
     application.include_router(chat_router, prefix=config.API_PREFIX)
@@ -105,10 +106,10 @@ def create_application() -> FastAPI:
 def register_exception_handlers(application: FastAPI) -> None:
     """
     Register global exception handlers.
-    
+
     Provides consistent error responses compatible with OpenAI API.
     """
-    
+
     @application.exception_handler(AppException)
     async def app_exception_handler(
         request: Request,