style: say which lint rules this repo keeps, then keep them

`[tool.ruff]` set a line length and a target version and nothing else, so which
rules ran depended on the installed ruff's defaults — fine while nobody ran it,
not fine for a CI job that can turn a push red because a tool changed its mind.

The set is named explicitly (E, F, I, UP, SIM, B, RUF) with three ignores that
are each a real decision rather than a shrug: B008, because FastAPI's whole
dependency idiom is a call in a default argument; E501, because wrapping prose
is a formatter's business; and RUF001, because the reports and the bot speak
Russian and "ambiguous character" is the wrong complaint about a string the
user is meant to read.

The 73 findings that remained are fixed — mostly `datetime.timezone.utc` →
`datetime.UTC`, `typing.Callable` → `collections.abc.Callable`, import order,
and unused `noqa` markers — plus seven by hand: `contextlib.suppress` for two
try/except/pass pairs, unpacking instead of list concatenation, one negated
return.

`ruff format` is deliberately *not* adopted: it would reformat 51 files, and
some of the layout here is hand-set (the comment tables, the DNS wire-format
structs) in ways a formatter flattens.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

backend/app/models.py

@@ -8,7 +8,7 @@ day so a re-send shows the same numbers.
 
 from __future__ import annotations
 
-from datetime import date, datetime, timezone
+from datetime import UTC, date, datetime
 from typing import Any
 
 from sqlalchemy import (
@@ -30,7 +30,7 @@ from app.db import Base
 
 
 def utcnow() -> datetime:
-    return datetime.now(timezone.utc)
+    return datetime.now(UTC)
 
 
 class SettingsRow(Base):
@@ -80,7 +80,7 @@ class PomodoroSession(Base):
     note: Mapped[str | None] = mapped_column(Text)
     abandon_reason: Mapped[str | None] = mapped_column(Text)
 
-    block_events: Mapped[list["BlockEvent"]] = relationship(back_populates="session")
+    block_events: Mapped[list[BlockEvent]] = relationship(back_populates="session")
 
     @property
     def is_work(self) -> bool: