add app url to federation

data/federations/bluesky/index.yaml

@@ -3,5 +3,6 @@ plc: https://plc.directory
 bgs: https://bsky.social
 appview: https://bsky.social
 pds: https://bsky.social
+app: https://app.bsky-sandbox.dev
 links:
   web: https://blueskyweb.xyz/

data/federations/sandbox/index.yaml

@@ -2,5 +2,6 @@ name: Bluesky Sandbox
 plc: https://plc.bsky-sandbox.dev
 bgs: https://bgs.bsky-sandbox.dev
 appview: https://api.bsky-sandbox.dev
+app: https://app.bsky-sandbox.dev
 links:
   web: https://bsky-sandbox.dev/

schema/federation.yaml

@@ -29,5 +29,8 @@ properties:
   pds:
     type: string
     format: url
+  app:
+    type: string
+    format: url
   links:
     $ref: '#/definitions/links'