feat: encrypt staging secrets

.gitignore

 .gitsecret/keys/random_seed
 !*.secret
+backend/k8s/overlays/stg/database.secrets.env
\ No newline at end of file

.gitsecret/paths/mapping.cfg

+backend/k8s/overlays/stg/database.secrets.env:b292610d7ad4825d3b934bb69aaa7b72f8cad412ef6c572a58c7f07b549ac206

backend/k8s/overlays/stg/kustomization.yaml

@@ -16,11 +16,7 @@ images:
     newName: gitlab.inf.unibz.it:4567/commul/oetzit/backend
     newTag: development
 secretGenerator:
-  # TODO: avoid using literals
-  - literals:
-      - username=db_user
-      - password=db_pass
-      - database=db_name
-      - url=postgres://db_user:db_pass@oetzit-database-service/db_name
+  - envs:
+      - database.secrets.env
     name: oetzit-database-credentials
     type: Opaque