Consolidate domain exceptions into app.exceptions

Move all shared domain exception classes to backend/app/exceptions.py and update services/routers to import the canonical exceptions. Update docs to reflect the shared exceptions source.

Docs/Tasks.md

@@ -90,6 +90,8 @@ Any config file write error in a filter operation (bad content, permission denie
 
 ### Task 4 — Consolidate all exception classes into app.exceptions
 
+**Status:** Completed
+
 **Severity:** High
 
 **Where:**