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/api/import_admin.py

@@ -1,6 +1,6 @@
 """Import admin API: trigger scan, list tasks, retry, clear."""
 
-from datetime import datetime, timedelta, timezone
+from datetime import UTC, datetime, timedelta
 
 from quart import Blueprint, jsonify, request
 from sqlalchemy import delete, select, update
@@ -137,7 +137,7 @@ async def clear_completed():
         return jsonify({"error": "status must be a list"}), 400
 
     cutoff = (
-        datetime.now(timezone.utc) - timedelta(days=int(age_days)) if age_days else None
+        datetime.now(UTC) - timedelta(days=int(age_days)) if age_days else None
     )
 
     Session = _session_factory()