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/data_sources.py

@@ -5,10 +5,11 @@ Provides a centralized panel for managing all external data sources
 including sync triggers, enable/disable toggles, and statistics.
 """
 
+from typing import Optional
+
 from fastapi import APIRouter, Depends
 from pydantic import BaseModel
 from sqlalchemy.orm import Session
-from typing import Optional
 
 from app.database import get_db
 from app.dependencies.auth import require_role
@@ -23,7 +24,6 @@ from app.services.data_source_service import (
     update_source,
 )
 
-
 # ---------------------------------------------------------------------------
 # Pydantic schemas for request validation
 # ---------------------------------------------------------------------------