Fix exception handler overlap issue - add DomainError catch-all handler

**Problem:** Broad exception handlers created fragility where adding a new
DomainError subclass without explicit registration would silently fall through
to the generic exception handler, losing the specific error_code and metadata.

**Solution:**
1. Import DomainError in main.py for explicit handler registration
2. Fix type hints in exception handlers from 'Exception' to specific types
   - NotFoundError handler now typed as 'NotFoundError'
   - BadRequestError handler now typed as 'BadRequestError'
   - ConflictError handler now typed as 'ConflictError'
   - DomainError handler now typed as 'DomainError'
   - ServiceUnavailableError handler now typed as 'ServiceUnavailableError'
3. Add DomainError as an explicit catch-all handler in the registration chain
   - Positioned after specific handlers, before HTTPException
   - Any unregistered DomainError subclass now gets correct error_code + metadata
4. Document the exception handler hierarchy with detailed comments
5. Update Backend-Development.md with handler hierarchy documentation
6. Update Architekture.md section 2.2 with exception handler details
7. Fix test expectations in test_main.py to verify ErrorResponse format

**Impact:** Any new DomainError subclass now automatically gets correct HTTP 500
status, error_code, and metadata - even if developer forgets explicit handler.

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

backend/tests/test_main.py

@@ -112,15 +112,25 @@ async def test_create_app_global_domain_exception_handlers() -> None:
     async with AsyncClient(transport=transport, base_url="http://test") as client:
         response = await client.get("/not-found")
         assert response.status_code == 404
-        assert response.json() == {"detail": "Jail not found: 'ssh'"}
+        data = response.json()
+        assert data["code"] == "jail_not_found"
+        assert data["detail"] == "Jail not found: 'ssh'"
+        assert data["metadata"] == {"jail_name": "ssh"}
+        assert "correlation_id" in data
 
         response = await client.get("/bad-request")
         assert response.status_code == 400
-        assert response.json() == {"detail": "invalid payload"}
+        data = response.json()
+        assert data["code"] == "config_validation_failed"
+        assert data["detail"] == "invalid payload"
+        assert "correlation_id" in data
 
         response = await client.get("/server-error")
         assert response.status_code == 500
-        assert response.json() == {"detail": "write failed"}
+        data = response.json()
+        assert data["code"] == "config_write_failed"
+        assert data["detail"] == "write failed"
+        assert "correlation_id" in data
 
 
 def test_create_app_disables_cors_by_default() -> None: