Add App__Domain config to derive connect redirect URI

Let deployments set a single App__Domain (e.g. sptb.example.com) instead of
spelling out the full ClientConnect__RedirectUri. The SharePoint-connect
callback is derived as <domain>/connect/callback; an explicit RedirectUri
still wins for back-compat.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

docker-compose.prebuilt.yml

@@ -15,6 +15,9 @@ services:
     environment:
       - ASPNETCORE_ENVIRONMENT=Production
       - DataFolder=/data
+      # Public domain the app is reached at (e.g. sptb.example.com). The SharePoint-connect
+      # redirect URI is derived from it as <domain>/connect/callback.
+      - App__Domain=${App__Domain:-}
       # OIDC config — overrides the placeholder values baked into appsettings.json.
       # Authority is derived from TenantId in code; do NOT set an Authority key.
       # Put real values in a .env file beside this compose file (NO quotes around