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>

tests/test_gallery_service.py

@@ -1,4 +1,4 @@
-from datetime import datetime, timedelta, timezone
+from datetime import UTC, datetime, timedelta
 
 import pytest
 
@@ -12,7 +12,7 @@ from backend.app.services.gallery_service import (
 
 
 def _now():
-    return datetime.now(timezone.utc)
+    return datetime.now(UTC)
 
 
 async def _seed_images(db, count: int, sha_prefix: str = "0") -> list[ImageRecord]:
@@ -37,7 +37,7 @@ async def _seed_images(db, count: int, sha_prefix: str = "0") -> list[ImageRecor
 
 
 def test_cursor_roundtrip():
-    ts = datetime(2026, 5, 14, 12, 30, 0, tzinfo=timezone.utc)
+    ts = datetime(2026, 5, 14, 12, 30, 0, tzinfo=UTC)
     encoded = encode_cursor(ts, 42)
     back_ts, back_id = decode_cursor(encoded)
     assert back_ts == ts