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/core/config.py

@@ -2,7 +2,7 @@ import os
 import secrets
 import warnings
 from pathlib import Path
-from typing import Annotated, Any, Literal
+from typing import Annotated, Any, Literal, Self
 
 from pydantic import (
     AnyUrl,
@@ -13,7 +13,6 @@ from pydantic import (
     model_validator,
 )
 from pydantic_settings import BaseSettings, SettingsConfigDict
-from typing_extensions import Self
 
 #: Everything the engine keeps on disk, relative to :attr:`Settings.DATA_DIR`.
 #: One setting to move the lot; each still overridable on its own, which is

backend/fluksio/core/security.py

@@ -1,6 +1,6 @@
 import base64
 import uuid
-from datetime import datetime, timedelta, timezone
+from datetime import UTC, datetime, timedelta
 from functools import lru_cache
 from typing import Any
 
@@ -40,7 +40,7 @@ PANEL_AUDIENCE = "fluksio-panel"
 
 
 def create_access_token(subject: str | Any, expires_delta: timedelta) -> str:
-    expire = datetime.now(timezone.utc) + expires_delta
+    expire = datetime.now(UTC) + expires_delta
     to_encode = {"exp": expire, "sub": str(subject)}
     encoded_jwt = jwt.encode(to_encode, settings.SECRET_KEY, algorithm=ALGORITHM)
     return encoded_jwt
@@ -109,7 +109,7 @@ def create_oauth_access_token(
     from someone's browser is refused there, so agent traffic never arrives
     looking like a person's.
     """
-    now = datetime.now(timezone.utc)
+    now = datetime.now(UTC)
     payload = {
         "sub": str(user_id),
         "iss": settings.oauth_issuer,
@@ -140,7 +140,7 @@ def create_worker_token(name: str, expires_delta: timedelta) -> str:
     worker's token grants no API access, and an agent's opens no worker
     connection.
     """
-    now = datetime.now(timezone.utc)
+    now = datetime.now(UTC)
     payload = {
         "sub": name,
         "iss": settings.oauth_issuer,
@@ -193,7 +193,7 @@ def create_panel_token(
     Long-lived on purpose: a wall tablet is set up once and left running, and
     it has no keyboard to log in again with.
     """
-    now = datetime.now(timezone.utc)
+    now = datetime.now(UTC)
     payload = {
         "sub": str(user_id),
         "aud": PANEL_AUDIENCE,

backend/fluksio/core/types.py

@@ -6,7 +6,7 @@ models cannot import from there.
 
 from __future__ import annotations
 
-from datetime import datetime, timezone
+from datetime import UTC, datetime
 from typing import Any
 
 from sqlalchemy import DateTime, TypeDecorator
@@ -31,8 +31,8 @@ class UTCDateTime(TypeDecorator[datetime]):
         if value is None:
             return None
         if value.tzinfo is None:
-            return value.replace(tzinfo=timezone.utc)
-        return value.astimezone(timezone.utc)
+            return value.replace(tzinfo=UTC)
+        return value.astimezone(UTC)
 
     def process_result_value(
         self, value: datetime | None, dialect: Any
@@ -40,5 +40,5 @@ class UTCDateTime(TypeDecorator[datetime]):
         if value is None:
             return None
         if value.tzinfo is None:
-            return value.replace(tzinfo=timezone.utc)
-        return value.astimezone(timezone.utc)
+            return value.replace(tzinfo=UTC)
+        return value.astimezone(UTC)