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!

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/services/campaign_scheduler_service.py

@@ -5,17 +5,16 @@ fresh tests, and computing the next run date.
 """
 
 import logging
-import uuid
 from datetime import datetime, timedelta
 
 from sqlalchemy.orm import Session
 
 from app.models.campaign import Campaign, CampaignTest
-from app.models.test import Test
 from app.models.enums import TestState
-from app.services.notification_service import create_notification
-from app.services.audit_service import log_action
+from app.models.test import Test
 from app.models.user import User
+from app.services.audit_service import log_action
+from app.services.notification_service import create_notification
 
 logger = logging.getLogger(__name__)
 
@@ -166,7 +165,10 @@ def check_and_run_recurring_campaigns(db: Session) -> int:
                     user_id=campaign.created_by,
                     type="recurring_campaign_run",
                     title="Recurring campaign executed",
-                    message=f'Campaign "{child.name}" was automatically created from recurring template "{campaign.name}".',
+                    message=(
+                        f'Campaign "{child.name}" was automatically created '
+                        f'from recurring template "{campaign.name}".'
+                    ),
                     entity_type="campaign",
                     entity_id=child.id,
                 )