fix(repo): correct mapping for domains (#10653)

This pull request fixes an issue where the repository would fail to scan
organization or instance structs if the `domains` column was `NULL`.

## Which problems are solved

If the `domains` column of `orgs` or `instances` was `NULL`, the
repository failed scanning into the structs. This happened because the
scanning mechanism did not correctly handle `NULL` JSONB columns.

## How the problems are solved

A new generic type `JSONArray[T]` is introduced, which implements the
`sql.Scanner` interface. This type can correctly scan JSON arrays from
the database, including handling `NULL` values gracefully.

The repositories for instances and organizations have been updated to
use this new type for the domains field. The SQL queries have also been
improved to use `FILTER` with `jsonb_agg` for better readability and
performance when aggregating domains.

## Additional changes
* An unnecessary cleanup step in the organization domain tests for
already removed domains has been removed.
* The `pgxscan` library has been replaced with `sqlscan` for scanning
`database/sql`.Rows.
* Minor cleanups in integration tests.

backend/v3/storage/database/repository/array.go

@@ -0,0 +1,21 @@
+package repository
+
+import (
+	"encoding/json"
+	"errors"
+)
+
+type JSONArray[T any] []*T
+
+func (a JSONArray[T]) Scan(src any) error {
+	switch s := src.(type) {
+	case string:
+		return json.Unmarshal([]byte(s), &a)
+	case []byte:
+		return json.Unmarshal(s, &a)
+	case nil:
+		return nil
+	default:
+		return errors.New("unsupported scan source")
+	}
+}