refactor(pep8): enforce full PEP8 compliance across backend Python codebase

- ruff.toml: select E/W/F/I/N rules, line-length=120, drop legacy ignores
- Auto-fix: sort 82 import blocks (isort), remove 29 unused imports,
  strip 6 trailing-whitespace blank lines in docstrings
- main.py: move setup_logging and settings imports to top (E402)
- errors.py: noqa N818 on DDD exception names (96 call sites, safe)
- intel_service.py: noqa N817 for universal ET alias
- atomic/elastic/sigma import services: move _MAX_UNCOMPRESSED_SIZE and
  _MAX_ENTRIES to module level (N806)
- compliance_import_service.py: move SAMPLE_CONTROLS / CIS_CONTROLS to
  module level; wrap long description strings (N806 + E501)
- snapshot_service.py: move STATUS_ORDER dict to module level (N806)
- sigma_import_service.py: remove dead dedup_key expression (F841)
- threat_actor_import_service.py: remove dead stix_to_actor expression (F841)
- data_source.py, seed_demo.py, campaign_scheduler_service.py,
  lolbas_import_service.py: wrap lines exceeding 120 chars (E501)
- d3fend_import_service.py: per-file E501 ignore (data file with long strings)

All 439 unit tests pass. ruff check app/ → All checks passed!

backend/app/services/atomic_import_service.py

@@ -24,7 +24,6 @@ templates are identified by their ``atomic_test_id`` and simply skipped.
 
 import io
 import logging
-import os
 import shutil
 import tempfile
 import zipfile
@@ -54,6 +53,10 @@ _DOWNLOAD_TIMEOUT = 300
 # Top-level directory name inside the ZIP
 _ZIP_ROOT_PREFIX = "atomic-red-team-master"
 
+# Safety limits for ZIP extraction — prevent zip-bomb DoS
+_MAX_UNCOMPRESSED_SIZE = 500 * 1024 * 1024  # 500 MB
+_MAX_ENTRIES = 50_000
+
 
 # ---------------------------------------------------------------------------
 # Internal helpers
@@ -77,11 +80,6 @@ def _safe_extract_zip(zip_bytes: bytes, dest: str) -> None:
     directory (path traversal / Zip Slip) or if the archive exceeds the
     safety limits.
     """
-    # Maximum uncompressed size: 500 MB — prevents zip-bomb DoS
-    _MAX_UNCOMPRESSED_SIZE = 500 * 1024 * 1024
-    # Maximum number of entries
-    _MAX_ENTRIES = 50_000
-
     dest_path = Path(dest).resolve()
 
     with zipfile.ZipFile(io.BytesIO(zip_bytes)) as zf: