Carry custom mood labels readable in a portable backup

The labels of the mood tags and mood categories an account created
travelled as the stored ciphertext in a portable file, so a restore onto
a host with another key refused the file or wrote labels nobody could
open. They now follow the cycle note: readable in a portable file, sealed
under the receiving host's key on restore, ciphertext kept only where
this host opens it and otherwise reported as moodLabelCiphertext.

The sealing helpers move to one module both sections share.

docs/api/openapi.yaml

@@ -36796,6 +36796,10 @@ components:
                 anyOf:
                   - type: string
                   - type: "null"
+              label:
+                anyOf:
+                  - type: string
+                  - type: "null"
             required:
               - id
               - key
@@ -36853,6 +36857,10 @@ components:
                 anyOf:
                   - type: string
                   - type: "null"
+              label:
+                anyOf:
+                  - type: string
+                  - type: "null"
               scaleMin:
                 default: 1
                 type: integer