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/nodes/base.py

@@ -10,7 +10,7 @@ import asyncio
 import inspect
 import logging
 from collections.abc import Callable, Coroutine, Iterable, Iterator
-from typing import TYPE_CHECKING, Any, TypeAlias
+from typing import TYPE_CHECKING, Any
 
 from fluksio.flow import logs
 from fluksio.flow.messages import MessageSpec, qualify
@@ -26,7 +26,7 @@ logger = logging.getLogger(__name__)
 
 # What a node hands back: the pipeline's state once it is bound, since a
 # trigger runs the graph, and its own outputs when it is not.
-NodeResult: TypeAlias = "StateBackend | dict[str, Any] | None"
+type NodeResult = StateBackend | dict[str, Any] | None
 
 
 class NodeOutputError(TypeError):

backend/fluksio/flow/nodes/delay.py

@@ -265,7 +265,7 @@ class DelayNode(Node):
                     await asyncio.wait_for(stop.wait(), timeout=wait_seconds)
                     # If we get here, stop was requested
                     break
-                except asyncio.TimeoutError:
+                except TimeoutError:
                     # Timeout means it's time to fire
                     pass
 

backend/fluksio/flow/nodes/inject.py

@@ -159,7 +159,7 @@ class InjectNode(Node):
         """Wait, but wake immediately if the node is being stopped."""
         try:
             await asyncio.wait_for(stop.wait(), timeout=seconds)
-        except asyncio.TimeoutError:
+        except TimeoutError:
             pass
 
     async def _fire(self) -> None: