refactor(types): add comprehensive type annotations across backend Python codebase

Enable ANN rules in ruff.toml (flake8-annotations) and resolve all 221 violations:

ANN201/ANN202 — return types on 168 public/private functions:
- All 28 FastAPI routers: endpoints annotated with dict/list/specific schema/
  StreamingResponse/FileResponse/JSONResponse as appropriate
- main.py: lifespan→AsyncGenerator[None,None], exception handlers→JSONResponse
- database.py: get_db→Generator[Session,None,None], proxy methods→correct types
- middleware/request_context.py: dispatch→Response with Callable call_next type

ANN001/ANN002/ANN003 — 32 missing argument types:
- seed_demo.py: all db parameters typed as Session
- domain/unit_of_work.py: __aexit__ exc_type/exc_val/exc_tb typed with TracebackType
- services: audit_service user_id→UUID|None, heatmap_service query/model/builder,
  notification_service test→Test, tempo_service test→Test/user→User,
  test_workflow_service test_id→UUID, campaign_crud **fields→object,
  test_crud **fields→object (4 sites)

ANN401 — 16 Any usages resolved:
- Domain entities (campaign/technique/threat_actor/test_entity): replaced Any with
  actual ORM types via TYPE_CHECKING guards to avoid circular imports
- detection_rule_service: test_id/detection_rule_id/evaluator_id→UUID
- score_cache: kept Any with # noqa: ANN401 (genuinely generic cache)
- jira_service/tempo_service: kept Any with # noqa: ANN401 (lazy optional deps)
- d3fend_import_service: _to_str(v: Any) kept with # noqa: ANN401

ANN204/ANN205/ANN206 — special/static/class methods:
- database.py proxy __call__/__getattr__: *args: object/**kwargs: object
- schemas/test.py model_validate: obj→object, **kwargs→object
- sa_technique_repository._int_type→type

All 439 unit tests pass. ruff check app/ → All checks passed!

backend/app/routers/system.py

@@ -30,7 +30,7 @@ def trigger_mitre_sync(
     request: Request,
     db: Session = Depends(get_db),
     current_user: User = Depends(require_role("admin")),
-):
+) -> dict:
     """Manually trigger a MITRE ATT&CK synchronisation.
 
     **Requires** the ``admin`` role.
@@ -50,7 +50,7 @@ def trigger_mitre_sync(
 def trigger_intel_scan(
     db: Session = Depends(get_db),
     current_user: User = Depends(require_role("admin")),
-):
+) -> dict:
     """Manually trigger a threat-intelligence scan.
 
     **Requires** the ``admin`` role.
@@ -71,7 +71,7 @@ def trigger_atomic_import(
     request: Request,
     db: Session = Depends(get_db),
     current_user: User = Depends(require_role("admin")),
-):
+) -> dict:
     """Trigger an import of Atomic Red Team tests as TestTemplates.
 
     **Requires** the ``admin`` role.
@@ -101,7 +101,7 @@ def trigger_atomic_import(
 @router.get("/scheduler-status")
 def scheduler_status(
     current_user: User = Depends(require_role("admin")),
-):
+) -> dict:
     """Return the current state of the background scheduler.
 
     **Requires** the ``admin`` role.