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/operational_metrics_service.py

@@ -6,14 +6,14 @@ Calculates security operations KPIs from test data and audit logs.
 from datetime import datetime, timedelta
 from typing import Optional
 
-from sqlalchemy import func, case, and_, or_, extract
+from sqlalchemy import func
 from sqlalchemy.orm import Session
 
-from app.models.test import Test
-from app.models.technique import Technique
-from app.models.test_detection_result import TestDetectionResult
 from app.models.audit import AuditLog
-from app.models.enums import TestState, TestResult
+from app.models.enums import TestResult, TestState
+from app.models.technique import Technique
+from app.models.test import Test
+from app.models.test_detection_result import TestDetectionResult
 
 
 def _safe_stats(values: list[float]) -> dict: