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/tasks/import_file.py

@@ -2,7 +2,7 @@
 updates the ImportTask state machine + ImportBatch counters atomically.
 """
 
-from datetime import datetime, timezone
+from datetime import UTC, datetime
 from pathlib import Path
 
 from sqlalchemy import create_engine, select, update
@@ -34,7 +34,7 @@ def import_media_file(self, import_task_id: int) -> dict:
             return {"status": "missing", "task_id": import_task_id}
 
         task.status = "processing"
-        task.started_at = datetime.now(timezone.utc)
+        task.started_at = datetime.now(UTC)
         session.add(task)
         session.commit()
 
@@ -55,7 +55,7 @@ def import_media_file(self, import_task_id: int) -> dict:
         except Exception as exc:  # pragma: no cover — pipeline crash
             task.status = "failed"
             task.error = f"{type(exc).__name__}: {exc}"
-            task.finished_at = datetime.now(timezone.utc)
+            task.finished_at = datetime.now(UTC)
             session.execute(
                 update(ImportBatch)
                 .where(ImportBatch.id == task.batch_id)
@@ -86,7 +86,7 @@ def import_media_file(self, import_task_id: int) -> dict:
             counter_col_name = "failed"
             counter_col = ImportBatch.failed
 
-        task.finished_at = datetime.now(timezone.utc)
+        task.finished_at = datetime.now(UTC)
         session.execute(
             update(ImportBatch)
             .where(ImportBatch.id == task.batch_id)
@@ -114,7 +114,7 @@ def import_media_file(self, import_task_id: int) -> dict:
                 .where(ImportBatch.id == task.batch_id)
                 .values(
                     status="complete",
-                    finished_at=datetime.now(timezone.utc),
+                    finished_at=datetime.now(UTC),
                 )
             )
             session.commit()