feat(crl): add CRL consumption and custom verifier for mTLS revocation enforcement

Implements agent-side CRL consumption for mTLS certificate revocation
checking, as specified in issue #20.

Changes:
- NEW: src/auth/crl.rs - CRL loading, parsing, signature verification,
  in-memory revoked serial index (HashSet), 24h background refresh task
- MODIFY: src/auth/mtls.rs - CrlAwareVerifier wrapping WebPkiClientVerifier
  with post-chain CRL serial lookup; fails closed on invalid signature,
  degrades gracefully when CRL is missing
- MODIFY: src/auth/mod.rs - Register crl module, re-export CrlState/CrlStatus
- MODIFY: src/config/loader.rs - Add crl_path field to TlsConfig
- MODIFY: src/main.rs - Load CRL on startup, spawn refresh task, wire
  SharedCrlState into server and health endpoint
- MODIFY: src/api/handlers/system.rs - Add crl_status and crl_age_seconds
  to health check response
- MODIFY: Cargo.toml - Add arc-swap, base64 deps; enable x509-parser
  verify feature for CRL signature verification

Design decisions:
- ArcSwap for lock-free atomic CRL state swaps on the hot path
- O(1) serial lookup via HashSet<String> of hex-encoded serials
- Stale CRL = continue serving + warn + health reports degraded
- Invalid CRL signature = refuse to start (fail-closed)
- Missing CRL = fall back to WebPKI-only (backward compatible)

Companion to PR #26 in linux-patch-manager (manager-side CRL generation)

Refs: #20

Cargo.lock

@@ -390,6 +390,15 @@ version = "1.0.102"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "7f202df86484c868dbad7eaa557ef785d5c66295e41b460ef922eca0723b842c"
 
+[[package]]
+name = "arc-swap"
+version = "1.9.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "6a3a1fd6f75306b68087b831f025c712524bcb19aad54e557b1129cfa0a2b207"
+dependencies = [
+ "rustversion",
+]
+
 [[package]]
 name = "arraydeque"
 version = "0.5.1"
@@ -1925,7 +1934,9 @@ dependencies = [
  "actix-web-actors",
  "addr",
  "anyhow",
+ "arc-swap",
  "async-channel",
+ "base64 0.22.1",
  "chrono",
  "clap",
  "config",
@@ -4271,6 +4282,7 @@ dependencies = [
  "lazy_static",
  "nom",
  "oid-registry",
+ "ring",
  "rusticata-macros",
  "thiserror 1.0.69",
  "time",