Implement centralized exception handling and validation

- Add custom exception classes for structured error handling
- Implement global exception handlers in FastAPI application
- Add comprehensive request/response validation
- Create exception contract tests for validation
- Update backend development documentation

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Docs/Tasks.md

@@ -1,22 +1,3 @@
-## 4) Module-level mutable runtime flags in service layer
-- Where found:
-	- [backend/app/services/jail_service.py](backend/app/services/jail_service.py)
-- Why this is needed:
-	- Global mutable state is difficult to reason about under concurrency and tests.
-- Goal:
-	- Move mutable runtime state to managed app/runtime state services.
-- What to do:
-	- Replace module-level flags with injected state holder.
-	- Guard mutations with clear synchronization boundaries.
-- Possible traps and issues:
-	- Race conditions can reappear if state updates are spread across modules.
-- Docs changes needed:
-	- Document allowed mutable-state locations.
-- Doc references:
-	- [Docs/Architekture.md](Docs/Architekture.md)
-
----
-
 ## 5) Inconsistent domain exception contracts across services
 - Where found:
 	- [backend/app/routers/jails.py](backend/app/routers/jails.py)