Add merchant-onboard scoped-key role for self-serve onboarding

New scoped API-key role granting read + products:write + policies:write +
licenses:write — the least-privilege credential for end-to-end catalog
setup and license issuance (create product, define policies/tiers, issue
licenses against them) without holding the master key.

The catalog write scopes already existed and were enforced on the
endpoints; only the role->scope expansion was missing. So this is a new
Role variant, not a scope-model change. grants() matches scope strings
explicitly (never by :write suffix) so the role can't widen into
settings / payment / merchant-profile / webhook writes, and every
master-only operation stays behind require_admin and so is structurally
unreachable. Existing tier caps still bound it (Creator: 5 products /
5 policies per product).

Migration 0023 rebuilds scoped_api_keys to widen the role CHECK (SQLite
can't alter a CHECK in place); the table has no FKs, so it's a plain
copy/drop/rename. Test covers the full onboard chain under the key's own
credential plus denial of master-only gates and support-only writes.

licensing-service/src/api/openapi.rs

@@ -50,7 +50,7 @@ const SPEC_JSON: &str = r##"{
       "bearerAuth": {
         "type": "http",
         "scheme": "bearer",
-        "description": "Master admin_api_key OR a scoped API key (ks_...). Scoped keys are gated on a role: read-only, license-issuer, support, or full-admin."
+        "description": "Master admin_api_key OR a scoped API key (ks_...). Scoped keys are gated on a role: read-only, license-issuer, support, merchant-onboard, or full-admin."
       }
     },
     "schemas": {
@@ -398,7 +398,7 @@ const SPEC_JSON: &str = r##"{
             "type": "object",
             "properties": {
               "label": { "type": "string", "description": "Operator-friendly name, e.g. 'Recap support bot'" },
-              "role":  { "type": "string", "enum": ["read-only", "license-issuer", "support", "full-admin"] }
+              "role":  { "type": "string", "enum": ["read-only", "license-issuer", "support", "merchant-onboard", "full-admin"] }
             },
             "required": ["label", "role"]
           } } }