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/routers/osint.py

@@ -4,7 +4,7 @@ OSINT items (CVEs, advisories, etc.) linked to techniques.
 
 from uuid import UUID
 
-from fastapi import APIRouter, Depends, Query, HTTPException, status
+from fastapi import APIRouter, Depends, HTTPException, Query, status
 from pydantic import BaseModel
 from sqlalchemy.orm import Session
 
@@ -17,9 +17,11 @@ from app.services.osint_enrichment_service import (
     get_osint_items_for_technique,
     get_osint_summary,
     get_technique_or_raise,
-    list_osint_items as service_list_osint_items,
     mark_osint_reviewed,
 )
+from app.services.osint_enrichment_service import (
+    list_osint_items as service_list_osint_items,
+)
 
 router = APIRouter(prefix="/osint", tags=["osint"])