fix(config): fail closed on POW_REQUIRED without FEATURE_POW; harden premium key check

- Reject boot when POW_REQUIRED (or the per-pubkey PoW list) is set while
  FEATURE_POW is off: the gate would silently never apply and /capabilities
  would report pow_required:false — a signup endpoint the operator believes
  is protected but isn't.
- Validate the premium PAYMENT_API_KEY through the same require_secret path
  the pay-to-register flow uses, so a placeholder key cannot reach production.
- Advertise the premium-post write policy in MessagingCapability and
  regenerate openapi.json to match.
- Give the premium integration fixtures unique per-run ids so the suite is
  re-runnable against a persistent local test database.

openapi.json

@@ -2968,7 +2968,7 @@
           },
           "write_policy": {
             "type": "string",
-            "description": "Write policy: `inbox-outbox` | `author-allowlist` | `open`."
+            "description": "Write policy: `inbox-outbox` | `author-allowlist` | `open` | `premium-post`."
           }
         }
       },