refactor(domain): introduce domain exceptions boundary

- Create domain/errors.py as canonical error hierarchy: DomainError, InvalidStateTransition, PermissionViolation, BusinessRuleViolation, EntityNotFoundError, DuplicateEntityError

- InvalidOperationError now inherits from BusinessRuleViolation for semantic consistency

- Convert domain/exceptions.py to backward-compatible re-export shim with legacy aliases (DomainException, InvalidTransitionError, AuthorizationError)

- Update error_handler.py to import from domain/errors.py and map all new error types

- Update main.py to register DomainError (new base) as the exception handler root

backend/app/main.py

@@ -38,7 +38,7 @@ from app.routers import professional_reports as professional_reports_router
 from app.routers import analytics as analytics_router
 from app.routers import advanced_metrics as advanced_metrics_router
 from app.routers import osint as osint_router
-from app.domain.exceptions import DomainException
+from app.domain.errors import DomainError
 from app.middleware.error_handler import domain_exception_handler
 from app.storage import ensure_bucket_exists
 from app.jobs.mitre_sync_job import start_scheduler, scheduler
@@ -77,7 +77,7 @@ app.state.limiter = limiter
 app.add_exception_handler(RateLimitExceeded, _rate_limit_exceeded_handler)
 
 # ── Domain exception → HTTP mapping ──────────────────────────────────────
-app.add_exception_handler(DomainException, domain_exception_handler)
+app.add_exception_handler(DomainError, domain_exception_handler)
 
 # ── CORS ──────────────────────────────────────────────────────────────────
 from app.config import settings as _settings