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/core/tracing_router.py

@@ -4,6 +4,8 @@ Trace viewer router.
 Serves the trace viewer UI and trace files when tracing is enabled.
 Only available when DEBUG=true.
 """
+
+from datetime import UTC
 from pathlib import Path
 
 from fastapi import APIRouter, HTTPException
@@ -66,12 +68,12 @@ async def list_traces(
         return {"traces": [], "total": 0}
 
     import json
-    from datetime import datetime, timezone, timedelta
+    from datetime import datetime, timedelta
 
     # Calculate cutoff time if filtering by time
     cutoff_time = None
     if since_minutes:
-        cutoff_time = datetime.now(timezone.utc) - timedelta(minutes=since_minutes)
+        cutoff_time = datetime.now(UTC) - timedelta(minutes=since_minutes)
 
     # Get all trace files, sorted by modification time (newest first)
     trace_files = sorted(
@@ -93,7 +95,7 @@ async def list_traces(
             trace_timestamp = data.get("timestamp")
             if cutoff_time and trace_timestamp:
                 try:
-                    ts = datetime.fromisoformat(trace_timestamp.replace('Z', '+00:00'))
+                    ts = datetime.fromisoformat(trace_timestamp.replace("Z", "+00:00"))
                     if ts < cutoff_time:
                         continue
                 except (ValueError, TypeError):
@@ -109,15 +111,17 @@ async def list_traces(
             if search and search.lower() not in request_preview.lower():
                 continue
 
-            traces.append({
-                "trace_id": data.get("trace_id"),
-                "timestamp": trace_timestamp,
-                "user": data.get("user"),
-                "status": trace_status,
-                "total_duration_ms": data.get("total_duration_ms"),
-                "span_count": len(data.get("spans", [])),
-                "request_preview": request_preview[:100],
-            })
+            traces.append(
+                {
+                    "trace_id": data.get("trace_id"),
+                    "timestamp": trace_timestamp,
+                    "user": data.get("user"),
+                    "status": trace_status,
+                    "total_duration_ms": data.get("total_duration_ms"),
+                    "span_count": len(data.get("spans", [])),
+                    "request_preview": request_preview[:100],
+                }
+            )
         except Exception as e:
             logger.warning("trace_list_parse_error", path=str(path), error=str(e))
 
@@ -145,6 +149,7 @@ async def get_trace(trace_id: str):
 
     try:
         import json
+
         with open(trace_path) as f:
             data = json.load(f)
         return JSONResponse(content=data)