fix(#2164): domain-URL webhooks + HMAC + config-PR polling fallback

Both webhook registrations (knowledge push + config-PR pull_request) now
use the public hive domain instead of loopback:
  https://<HYPERHIVE_HIVE_DOMAIN>/webhook/{knowledge,config-pr}

This routes deliveries through the gateway, bypassing the Forgejo SSRF
guard that blocked loopback delivery and silently broke the config-PR
merge flow since launch.

Changes:
- webhook_secret: new module — auto-generate + persist a 32-byte HMAC
  secret to STATE_ROOT/webhook-secret on first startup; verify
  X-Hub-Signature-256 on every incoming webhook POST (HMAC-SHA256).
- forge/mod.rs: ensure_config_pr_webhook now takes hive_domain +
  webhook_secret; sets secret in Forgejo hook config.
- workers/knowledge.rs: ensure_webhook same update.
- dashboard/webhook.rs: both handlers read raw Bytes first, verify HMAC,
  then parse JSON. Returns 401 on signature mismatch.
- dashboard/mod.rs: AppState carries webhook_secret; serve() takes it.
- main.rs: load/generate secret at startup; pass to registration tasks
  + dashboard; add 5-minute config-PR polling fallback task.
- forge/config_pr_poll.rs: new — scan agent-configs/* for open PRs with
  no pending MergeConfigPr approval; queue them. Idempotent.
- stores/approvals.rs: has_pending_merge_config_pr() for poll dedup.
- nix/modules/hive-gateway.nix: remove dashboardAuth from /webhook/
  location (HMAC replaces basic auth for webhook endpoints; Forgejo
  cannot send HTTP Basic credentials with webhook deliveries).

hive-c0re/src/dashboard/webhook.rs

@@ -6,20 +6,38 @@
 //!   repo queue a [`hive_sh4re::ApprovalKind::MergeConfigPr`] approval row
 //!   so the operator can review + approve the merge from the dashboard.
 //!
-//! Both endpoints are loopback-only (the axum listener binds
-//! `127.0.0.1:<port>`) and have no signature verification (the risk is low:
-//! loopback access implies host compromise already, and the config-PR path
-//! still requires the operator to approve on the dashboard).
+//! Both endpoints are reached via the gateway (HTTPS, public domain URL) so
+//! Forgejo's SSRF guard does not block delivery. Each delivery is verified
+//! against the `X-Hub-Signature-256` HMAC header Forgejo attaches; the
+//! shared secret is auto-generated at startup and persisted to
+//! [`crate::paths::webhook_secret_file()`].
 
 use axum::{
+    body::Bytes,
     extract::State,
-    http::StatusCode,
+    http::{HeaderMap, StatusCode},
     response::{IntoResponse, Response},
 };
 use serde::Deserialize;
 
 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.
+fn verify_hmac(state: &AppState, headers: &HeaderMap, body: &Bytes) -> Result<(), String> {
+    let sig = headers
+        .get("x-hub-signature-256")
+        .and_then(|v| v.to_str().ok())
+        .unwrap_or("");
+    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())
+}
+
 // ── knowledge webhook ──────────────────────────────────────────────────────────
 
 /// Minimal Forgejo push-webhook payload — only the fields we care about.
@@ -40,14 +58,31 @@ pub(super) struct PushWebhookRepo {
 /// agents see up-to-date documents on their next turn.
 ///
 /// Expected Forgejo webhook configuration:
-/// - URL: `http://127.0.0.1:<dashboard_port>/webhook/knowledge`
+/// - URL: `https://<HYPERHIVE_HIVE_DOMAIN>/webhook/knowledge`
+/// - Content type: `application/json`
 /// - Event: "Push" (fires on merge commits to main as well)
+/// - Secret: auto-generated HMAC key (see [`crate::webhook_secret`])
 ///
-/// No signature verification for now; the endpoint is loopback-only
-/// and only triggers a read-only `git pull` on an operator-curated repo.
+/// The gateway routes `/webhook/` → hive-c0re; the HMAC secret protects
+/// the endpoint from unauthenticated callers.
 pub(super) async fn post_webhook_knowledge(
-    axum::extract::Json(payload): axum::extract::Json<PushWebhookPayload>,
+    State(state): State<AppState>,
+    headers: HeaderMap,
+    body: Bytes,
 ) -> 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 payload = match serde_json::from_slice::<PushWebhookPayload>(&body) {
+        Ok(p) => p,
+        Err(e) => {
+            tracing::warn!("webhook/knowledge: JSON parse error: {e}");
+            return (StatusCode::BAD_REQUEST, "invalid JSON").into_response();
+        }
+    };
+
     let expected_repo = format!("{}/{}", crate::knowledge::ORG, crate::knowledge::REPO);
     let full_name = payload
         .repository
@@ -124,17 +159,32 @@ struct PrWebhookRepo {
 /// retry the delivery. Failures are logged at `warn` level.
 ///
 /// Expected Forgejo webhook configuration:
-/// - URL: `http://127.0.0.1:<dashboard_port>/webhook/config-pr`
+/// - URL: `https://<HYPERHIVE_HIVE_DOMAIN>/webhook/config-pr`
 /// - Content type: `application/json`
 /// - Events: "Pull Request" only
+/// - Secret: auto-generated HMAC key (see [`crate::webhook_secret`])
 /// - Organisation: `agent-configs` (org-level hook covers all config repos)
 ///
 /// hive-c0re registers this hook automatically at startup via
 /// [`crate::forge::ensure_config_pr_webhook`].
 pub(super) async fn post_webhook_config_pr(
     State(state): State<AppState>,
-    axum::extract::Json(payload): axum::extract::Json<PrWebhookPayload>,
+    headers: HeaderMap,
+    body: Bytes,
 ) -> 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 payload = match serde_json::from_slice::<PrWebhookPayload>(&body) {
+        Ok(p) => p,
+        Err(e) => {
+            tracing::warn!("webhook/config-pr: JSON parse error: {e}");
+            return (StatusCode::BAD_REQUEST, "invalid JSON").into_response();
+        }
+    };
+
     let action = payload.action.as_deref().unwrap_or("");
     // Only act on newly-opened or force-updated PRs.
     if action != "opened" && action != "synchronize" {