Fix the CI gates: Python 3.13, concurrency groups, hook violations

The gates have never gone green on the new runners. Three separate reasons:

- backend/Dockerfile shipped Python 3.10 while the code imports typing.Self
  and datetime.UTC, so the container exited on import and the suite could not
  even load its conftest. The image moves to 3.13 and the packages declare
  >=3.12, which is the floor the tests actually pass on; ruff's target follows
  and rewrites timezone.utc and asyncio.TimeoutError accordingly. Relocking
  drops the 3.10 branch, which bumps FastAPI and so regenerates the SDK.
- frontend/README.md had no trailing newline and two dashboard widgets used
  arbitrary text-[…] sizes. Both are em-relative on purpose, so they move to
  the inline style the neighbouring ramp already uses.
- Every commit left its own run queued: without a concurrency group a runner
  that was offline for a while works through a backlog nobody reads. A stack
  that fails to come up now prints its logs before the teardown removes it.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

backend/fluksio/flow/runs.py

@@ -35,7 +35,7 @@ import time
 import uuid
 from collections.abc import Callable
 from concurrent.futures import ThreadPoolExecutor
-from datetime import datetime, timedelta, timezone
+from datetime import UTC, datetime, timedelta
 from typing import Any
 
 from sqlalchemy import update
@@ -380,7 +380,7 @@ class RunService:
             cause=cause,
             status="queued",
             labels=required_labels(flow),
-            created_at=datetime.now(timezone.utc),
+            created_at=datetime.now(UTC),
             actor=actor,
         )
         with Session(db_engine) as session:
@@ -428,7 +428,7 @@ class RunService:
                 .where(col(Run.id) == run_id, col(Run.status) == "queued")
                 .values(
                     status="cancelled",
-                    finished_at=datetime.now(timezone.utc),
+                    finished_at=datetime.now(UTC),
                     status_reason="Cancelled before it started",
                 )
             )
@@ -516,7 +516,7 @@ class RunService:
                 break
             with self._lock:
                 mine = list(self._active)
-            now = datetime.now(timezone.utc)
+            now = datetime.now(UTC)
             try:
                 if mine:
                     with Session(db_engine) as session:
@@ -561,7 +561,7 @@ class RunService:
         The compare-and-swap is what makes a redelivered item harmless — the
         second engine to arrive updates nothing and walks away.
         """
-        now = datetime.now(timezone.utc)
+        now = datetime.now(UTC)
         with Session(db_engine) as session:
             result = session.exec(
                 update(Run)
@@ -648,7 +648,7 @@ class RunService:
             run_id=run_id,
             node=outcome.node[:255],
             status="ok" if outcome.ok else "error",
-            started_at=datetime.now(timezone.utc),
+            started_at=datetime.now(UTC),
             duration_ms=outcome.duration_ms,
             error=outcome.error[:ERROR_CAP],
             logs=outcome.logs[:LOG_CAP],
@@ -693,7 +693,7 @@ class RunService:
                         status_reason=reason,
                         result=result,
                         duration_ms=duration_ms,
-                        finished_at=datetime.now(timezone.utc),
+                        finished_at=datetime.now(UTC),
                     )
                 )
                 session.commit()