fix(#2164): address argus review — Option<String> secret + stale hook cleanup

Two issues flagged by argus in PR #2388 review:

1. Empty-key fallback: when load_or_generate() failed, webhook_secret was
   String::new(). An attacker knowing this could forge deliveries with a
   valid HMAC of the empty key. Fix: change to Option<String>; on None,
   skip hook registration entirely and return 503 from /webhook/* handlers
   (rather than 401 with a misleadingly-verifiable empty-key HMAC).

2. Stale hook cleanup: on upgrade from old code, old loopback hooks
   (http://127.0.0.1:.../webhook/knowledge, .../webhook/config-pr) were
   left alongside the new domain-URL hook. Fix: during ensure_webhook /
   ensure_config_pr_webhook, after listing hooks, delete any that end with
   our path suffix but point at a different base URL.

clippy + nix fmt clean.

hive-c0re/src/dashboard/mod.rs

@@ -58,7 +58,9 @@ struct AppState {
     coord: Arc<Coordinator>,
     /// HMAC-SHA256 secret shared with Forgejo webhook registrations.
     /// Verified on every incoming `/webhook/*` POST.
-    webhook_secret: String,
+    /// `None` when the secret could not be loaded at startup — all
+    /// `/webhook/*` requests are rejected with 503 in that case.
+    webhook_secret: Option<String>,
 }
 
 #[allow(
@@ -68,7 +70,11 @@ struct AppState {
               handler; splitting that exhaustive list across helpers would \
               obscure the route map for no readability gain"
 )]
-pub async fn serve(port: u16, coord: Arc<Coordinator>, webhook_secret: String) -> Result<()> {
+pub async fn serve(
+    port: u16,
+    coord: Arc<Coordinator>,
+    webhook_secret: Option<String>,
+) -> Result<()> {
     // API-only: the gateway static-serves the dashboard dist and proxies
     // non-static requests here (see hive-gateway.nix). Unmatched paths 404.
     let app = Router::new()

hive-c0re/src/dashboard/webhook.rs

@@ -25,8 +25,13 @@ use super::AppState;
 // ── HMAC helper ───────────────────────────────────────────────────────────────
 
 /// Verify the `X-Hub-Signature-256` header on an incoming Forgejo webhook.
-/// Returns `Err` (with a safe-to-log message) on mismatch or missing header.
+/// Returns `Err` (with a safe-to-log message) on mismatch, missing header,
+/// or when the HMAC secret is unavailable (load failure at startup).
 fn verify_hmac(state: &AppState, headers: &HeaderMap, body: &Bytes) -> Result<(), String> {
+    let secret = state
+        .webhook_secret
+        .as_deref()
+        .ok_or_else(|| "webhook HMAC secret unavailable; endpoint disabled".to_owned())?;
     let sig = headers
         .get("x-hub-signature-256")
         .and_then(|v| v.to_str().ok())
@@ -34,8 +39,7 @@ fn verify_hmac(state: &AppState, headers: &HeaderMap, body: &Bytes) -> Result<()
     if sig.is_empty() {
         return Err("missing X-Hub-Signature-256 header".to_owned());
     }
-    crate::webhook_secret::verify_signature(&state.webhook_secret, body, sig)
-        .map_err(|e| e.to_string())
+    crate::webhook_secret::verify_signature(secret, body, sig).map_err(|e| e.to_string())
 }
 
 // ── knowledge webhook ──────────────────────────────────────────────────────────
@@ -72,7 +76,12 @@ pub(super) async fn post_webhook_knowledge(
 ) -> Response {
     if let Err(e) = verify_hmac(&state, &headers, &body) {
         tracing::warn!("webhook/knowledge: HMAC verification failed: {e}");
-        return (StatusCode::UNAUTHORIZED, "signature mismatch").into_response();
+        let status = if e.contains("unavailable") {
+            StatusCode::SERVICE_UNAVAILABLE
+        } else {
+            StatusCode::UNAUTHORIZED
+        };
+        return (status, e).into_response();
     }
 
     let payload = match serde_json::from_slice::<PushWebhookPayload>(&body) {
@@ -174,7 +183,12 @@ pub(super) async fn post_webhook_config_pr(
 ) -> Response {
     if let Err(e) = verify_hmac(&state, &headers, &body) {
         tracing::warn!("webhook/config-pr: HMAC verification failed: {e}");
-        return (StatusCode::UNAUTHORIZED, "signature mismatch").into_response();
+        let status = if e.contains("unavailable") {
+            StatusCode::SERVICE_UNAVAILABLE
+        } else {
+            StatusCode::UNAUTHORIZED
+        };
+        return (status, e).into_response();
     }
 
     let payload = match serde_json::from_slice::<PrWebhookPayload>(&body) {