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_api_import_admin.py

@@ -1,4 +1,4 @@
-from datetime import datetime, timedelta, timezone
+from datetime import UTC, datetime, timedelta
 
 import pytest
 
@@ -46,7 +46,7 @@ async def test_list_tasks_with_status_filter(client, db):
     for status in ("complete", "failed", "complete"):
         db.add(ImportTask(
             batch_id=batch.id, source_path="/x", task_type="media", status=status,
-            finished_at=datetime.now(timezone.utc),
+            finished_at=datetime.now(UTC),
         ))
     await db.commit()
 
@@ -74,14 +74,14 @@ async def test_clear_completed(client, db):
     batch = ImportBatch(triggered_by="manual", source_path="/import", scan_mode="quick")
     db.add(batch)
     await db.flush()
-    long_ago = datetime.now(timezone.utc) - timedelta(days=30)
+    long_ago = datetime.now(UTC) - timedelta(days=30)
     db.add(ImportTask(
         batch_id=batch.id, source_path="/x", task_type="media",
         status="complete", finished_at=long_ago,
     ))
     db.add(ImportTask(
         batch_id=batch.id, source_path="/y", task_type="media",
-        status="complete", finished_at=datetime.now(timezone.utc),
+        status="complete", finished_at=datetime.now(UTC),
     ))
     await db.commit()