Make subscription_type, username, and years optional on POST /v1/admin/users

- subscription_type defaults to lifetime when omitted; validated when provided
- years is only required (and enforced) when subscription_type is yearly
- username uniqueness check and validation are skipped when username is empty
- Update OpenAPI spec to reflect pubkey as the only required field

internal/http/docs/openapi.yaml

@@ -222,12 +222,12 @@ paths:
           application/json:
             schema:
               type: object
-              required: [pubkey, username, subscription_type]
+              required: [pubkey]
               properties:
                 pubkey: { type: string }
-                username: { type: string }
-                subscription_type: { type: string, enum: [yearly, lifetime] }
-                years: { type: integer }
+                username: { type: string, description: "Optional NIP-05 username to assign" }
+                subscription_type: { type: string, enum: [yearly, lifetime], description: "Defaults to lifetime when omitted" }
+                years: { type: integer, description: "Required when subscription_type is yearly" }
       responses:
         '201':
           description: Created