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/middleware/error_handler.py

@@ -1,41 +1,43 @@
-"""Domain exception → HTTP response mapping.
+"""Domain error → HTTP response mapping.
 
 This module provides a single exception handler that converts
-domain-layer exceptions into structured JSON responses, keeping
+domain-layer errors into structured JSON responses, keeping
 the service layer free from FastAPI's ``HTTPException``.
 """
 
 from fastapi import Request
 from fastapi.responses import JSONResponse
 
-from app.domain.exceptions import (
-    AuthorizationError,
-    DomainException,
+from app.domain.errors import (
+    BusinessRuleViolation,
+    DomainError,
     DuplicateEntityError,
     EntityNotFoundError,
     InvalidOperationError,
-    InvalidTransitionError,
+    InvalidStateTransition,
+    PermissionViolation,
 )
 
-EXCEPTION_STATUS_MAP: dict[type[DomainException], int] = {
+EXCEPTION_STATUS_MAP: dict[type[DomainError], int] = {
     EntityNotFoundError: 404,
     DuplicateEntityError: 409,
-    InvalidTransitionError: 400,
+    InvalidStateTransition: 400,
     InvalidOperationError: 400,
-    AuthorizationError: 403,
+    BusinessRuleViolation: 400,
+    PermissionViolation: 403,
 }
 
 
 async def domain_exception_handler(
     request: Request,
-    exc: DomainException,
+    exc: DomainError,
 ) -> JSONResponse:
-    """Convert a :class:`DomainException` into a JSON error response."""
+    """Convert a :class:`DomainError` into a JSON error response."""
     status_code = EXCEPTION_STATUS_MAP.get(type(exc), 400)
 
     content: dict = {"detail": exc.message, "code": exc.code}
 
-    if isinstance(exc, InvalidTransitionError):
+    if isinstance(exc, InvalidStateTransition):
         content["current_state"] = exc.current_state
         content["target_state"] = exc.target_state
         content["valid_transitions"] = exc.valid_transitions