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/tests/test_metrics.py

@@ -6,7 +6,7 @@ the database, and writing to it is this module's whole job.
 
 import asyncio
 import time
-from datetime import datetime, timezone
+from datetime import UTC, datetime
 
 from sqlmodel import Session, select
 
@@ -71,7 +71,7 @@ def test_events_become_rollups_failures_runs_and_audit(db: Session) -> None:
     collector = MetricsCollector(EventBus())
     # The current minute, pinned: the second flush has to land in the same
     # bucket, and anything past the retention window is pruned on write.
-    ts = datetime.now(timezone.utc).replace(second=0, microsecond=0).timestamp()
+    ts = datetime.now(UTC).replace(second=0, microsecond=0).timestamp()
     _events(collector, ts, "1-0")
     collector.handle(
         {
@@ -130,7 +130,7 @@ def test_a_flush_the_database_refused_is_written_by_the_next_one(
     db: Session, monkeypatch
 ) -> None:
     collector = MetricsCollector(EventBus())
-    ts = datetime.now(timezone.utc).replace(second=0, microsecond=0).timestamp()
+    ts = datetime.now(UTC).replace(second=0, microsecond=0).timestamp()
     collector.handle(
         {
             "type": "audit",
@@ -157,7 +157,7 @@ def test_a_flush_the_database_refused_is_written_by_the_next_one(
 
 def test_a_node_id_wider_than_the_column_still_records(db: Session) -> None:
     collector = MetricsCollector(EventBus())
-    ts = datetime.now(timezone.utc).replace(second=0, microsecond=0).timestamp()
+    ts = datetime.now(UTC).replace(second=0, microsecond=0).timestamp()
     long_node = f"{FLOW}.{'w' * 400}"
     collector.handle(
         {