fix(fc2a): satisfy ruff 0.15.13 lint — UP017, UP042, I001

Ruff lint surfaced 23 violations across three rules; all addressed:

UP017 (Use datetime.UTC alias):
  Replaced 13 sites of datetime.now(timezone.utc) with datetime.now(UTC),
  also adjusted from-imports accordingly. UTC is a Python 3.11+ alias for
  timezone.utc that ruff's pyupgrade rules prefer.

UP042 (StrEnum):
  Replaced `class TagKind(str, Enum)` and `class SkipReason(str, Enum)`
  with `class Foo(StrEnum)`. StrEnum was added in Python 3.11 stdlib and
  is the modern idiom. Behavior is equivalent for our usage (the .value
  attribute, str(member) semantics).

I001 (Import sorting):
  Added `known-first-party = ["backend"]` to ruff.toml's [lint.isort] so
  ruff groups `backend.*` imports correctly. Without it, ruff treated
  them as third-party and demanded a different grouping. The existing
  import order is stdlib → third-party → first-party → local relative,
  which ruff now accepts.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/app/models/tag.py

@@ -7,7 +7,7 @@ different kinds and the same character name can exist in different fandoms.
 """
 
 from datetime import datetime
-from enum import Enum
+from enum import StrEnum
 
 from sqlalchemy import (
     CheckConstraint,
@@ -25,7 +25,7 @@ from sqlalchemy.orm import Mapped, mapped_column, relationship
 from .base import Base
 
 
-class TagKind(str, Enum):
+class TagKind(StrEnum):
     artist = "artist"
     character = "character"
     fandom = "fandom"