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/Architekture.md

@@ -85,7 +85,7 @@ backend/
 │   ├── `main.py`                # FastAPI app factory, lifespan, exception handlers
 │   ├── `config.py`              # Pydantic settings (env vars, .env loading)
 │   ├── `db.py`                  # Database connection and initialization
-│   ├── `exceptions.py`          # Shared domain exception classes
+│   ├── `exceptions.py`          # Shared domain exception classes; all services and routers import from here
 │   ├── `dependencies.py`        # FastAPI Depends() providers (DB, services, auth)
 │   ├── `models/`                # Pydantic schemas
 │   │   ├── auth.py            #   Login request/response, session models

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:**