Cargo.toml: disambiguate bin-target for cargo-leptos

The new question_lint/backfill_events [[bin]] targets left cargo-leptos
unable to tell which of the three binaries is the actual app server -
broke the last two deploy runs ("Several bin targets found for member
portal"). bin-target = "portal" fixes it; confirmed locally that
`cargo leptos build` gets past config resolution now.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Cargo.toml

@@ -121,6 +121,10 @@ codegen-units = 1
 
 [package.metadata.leptos]
 output-name = "portal"
+# Disambiguates the app binary from the two utility [[bin]] targets
+# (question_lint, backfill_events) added alongside it - cargo-leptos
+# otherwise can't tell which of the three is "the" server.
+bin-target = "portal"
 site-root = "target/site"
 site-pkg-dir = "pkg"
 style-file = "style/main.css"