feat(pki): add CRL generation, distribution endpoint, and enrollment bundle extension (#26)

* feat(pki): add CRL generation, distribution endpoint, and enrollment bundle extension

Implements manager-side CRL infrastructure for issue #7:
- Add CertAuthority::generate_crl() using rcgen 0.13
- Add GET /api/v1/pki/crl.pem public endpoint
- Extend PkiBundle with ca_chain and crl_pem fields
- Update enrollment route to include CRL in bundle
- Mount pki route as public endpoint
- Add proptest dev-dependency

* style: fix cargo fmt in enrollment.rs

---------

Co-authored-by: Draco Lunaris <331325+Draco-Lunaris@users.noreply.github.com>

crates/pm-web/src/routes/pki.rs

@@ -0,0 +1,61 @@
+//! PKI endpoints for certificate revocation list (CRL) distribution.
+//!
+//! This module exposes the CRL endpoint that agents poll every 24 hours to
+//! check for revoked certificates. The CRL is signed by the internal CA and
+//! is publicly accessible (CRLs are self-authenticating — they carry the CA
+//! signature and do not require client authentication).
+
+use crate::AppState;
+use axum::{
+    extract::State,
+    http::{header, StatusCode},
+    response::IntoResponse,
+    routing::get,
+    Router,
+};
+
+/// Define public PKI routes.
+///
+/// These endpoints are **unauthenticated** because CRLs are self-authenticating:
+/// the agent verifies the CRL signature against its pinned CA certificate.
+/// No client certificate or API key is required.
+pub fn router() -> Router<AppState> {
+    Router::new().route("/pki/crl.pem", get(get_crl))
+}
+
+/// `GET /api/v1/pki/crl.pem`
+///
+/// Returns the current Certificate Revocation List (CRL) as a PEM-encoded
+/// X.509 CRL. The CRL is signed by the internal CA and contains the serial
+/// numbers of all revoked certificates that have not yet expired.
+///
+/// # Cache headers
+///
+/// The response includes `Cache-Control: max-age=3600` (1 hour) to allow
+/// intermediate caches to serve the CRL. Agents refresh every 24 hours,
+/// so a 1-hour cache is a reasonable balance between freshness and load.
+///
+/// # CRL generation
+///
+/// The CRL is generated on demand from the `certificates` table. For our
+/// target scale (max ~2500 clients), this is a fast query and the resulting
+/// CRL is KB-range. If performance becomes a concern, the CRL can be cached
+/// in memory and regenerated on a schedule (see background task in main.rs).
+async fn get_crl(State(state): State<AppState>) -> impl IntoResponse {
+    match state.ca.generate_crl(&state.db).await {
+        Ok(crl_pem) => (
+            StatusCode::OK,
+            [(
+                header::CONTENT_TYPE,
+                "application/x-pem-file; charset=utf-8",
+            )],
+            [(header::CACHE_CONTROL, "max-age=3600")],
+            crl_pem,
+        )
+            .into_response(),
+        Err(e) => {
+            tracing::error!(error = %e, "Failed to generate CRL");
+            (StatusCode::INTERNAL_SERVER_ERROR, "Failed to generate CRL").into_response()
+        },
+    }
+}