chore(store): repin the official plugin-index signing key

The bootstrap key generated during implementation was lost, and its
replacement was pasted into the wrong repository's secret store — Gitea
secrets neither cross repos nor read back, so neither is usable. The private
half of this one lives in punktfunk-plugin-index's own INDEX_SIGNING_KEY,
which is the only thing that signs the catalog.

Nothing has shipped pinning any of these, so this is a straight replacement
of slot 0; the second slot stays reserved for a genuine rotation.

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

crates/punktfunk-host/src/store/sources.rs

@@ -33,7 +33,7 @@ pub(crate) const OFFICIAL_URL: &str =
 /// new key, ship a host that trusts both, retire the old one" instead of a flag day where old
 /// hosts lose the catalog. An empty slot is ignored.
 pub(crate) const OFFICIAL_KEYS: [&str; 2] = [
-    "ed25519:n/KgBQSSDZqvyGHa8PkHWHZtV1zRXLk0BdQUi4/BD/w=",
+    "ed25519:V7KKMg8sq2A2TW7D/GFWaM0ruAvigpld9r93JdWcQHw=",
     "", // rotation slot
 ];