Add public GET /v1/whitelist/pubkeys for active subscriber pubkeys

Expose JSON array of hex pubkeys backed by ListActivePubkeys query.
Includes OpenAPI documentation and integration tests.

Made-with: Love

internal/http/docs/openapi.yaml

@@ -108,6 +108,19 @@ paths:
           content:
             application/json:
               schema: { $ref: '#/components/schemas/Pricing' }
+  /v1/whitelist/pubkeys:
+    get:
+      tags: [Public]
+      summary: Active whitelist pubkeys
+      description: Hex-encoded pubkeys for all active (whitelisted) subscribers.
+      responses:
+        '200':
+          description: Pubkey list
+          content:
+            application/json:
+              schema:
+                type: array
+                items: { type: string, description: Hex pubkey }
   /v1/invoices:
     post:
       tags: [User]