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/snapshot_service.py

@@ -16,9 +16,9 @@ from sqlalchemy import func
 from sqlalchemy.orm import Session
 
 from app.domain.errors import EntityNotFoundError
-from app.models.technique import Technique
 from app.models.coverage_snapshot import CoverageSnapshot, SnapshotTechniqueState
 from app.models.enums import TechniqueStatus
+from app.models.technique import Technique
 from app.services.scoring_service import (
     bulk_technique_scores,
     calculate_organization_score,
@@ -26,6 +26,15 @@ from app.services.scoring_service import (
 
 logger = logging.getLogger(__name__)
 
+# Coverage status ordering for snapshot delta comparisons (higher = better coverage)
+_STATUS_ORDER: dict[str, int] = {
+    "not_evaluated": 0,
+    "not_covered": 1,
+    "in_progress": 2,
+    "partial": 3,
+    "validated": 4,
+}
+
 
 # ---------------------------------------------------------------------------
 # Serialization and queries
@@ -296,15 +305,6 @@ def compare_snapshots(
         .all()
     }
 
-    # Status priority for comparison
-    STATUS_ORDER = {
-        "not_evaluated": 0,
-        "not_covered": 1,
-        "in_progress": 2,
-        "partial": 3,
-        "validated": 4,
-    }
-
     improved = []
     worsened = []
     unchanged_count = 0
@@ -315,8 +315,8 @@ def compare_snapshots(
         a = states_a.get(mitre_id, {"status": "not_evaluated", "score": 0})
         b = states_b.get(mitre_id, {"status": "not_evaluated", "score": 0})
 
-        a_order = STATUS_ORDER.get(a["status"], 0)
-        b_order = STATUS_ORDER.get(b["status"], 0)
+        a_order = _STATUS_ORDER.get(a["status"], 0)
+        b_order = _STATUS_ORDER.get(b["status"], 0)
 
         if b_order > a_order or (b_order == a_order and b["score"] > a["score"]):
             improved.append({