fix(security): harden enrollment PKI bundle retrieval (#12)

- Add single-retrieval semantics: approved PKI bundles are atomically
  removed from the in-memory cache on first retrieval via DashMap::remove(),
  preventing concurrent requests from obtaining the private key
- Add TTL expiry: ApprovedEntry wraps PkiBundle with approved_at and ttl
  fields; bundles expire after ENROLLMENT_BUNDLE_TTL_SECS (600s / 10 min)
- Replace brute-force clear() purge with TTL-based retain() in background
  task, running every 60s instead of every 600s
- Audit tracing calls: confirm no raw polling token is logged; add security
  comment documenting this policy
- Document CSR-based enrollment as future enhancement in both enrollment.rs
  and SECURITY.md, explaining why server-generated keys are used currently

SECURITY.md

@@ -41,6 +41,28 @@ This project is a security tool — we hold ourselves to a high standard:
 - **CI enforcement**: All PRs require passing CI checks (fmt, clippy, test, audit, build)
 - **Dependency auditing**: `cargo audit` runs in CI to catch known vulnerabilities
 
+## Enrollment PKI Design Decisions
+
+### Server-Generated Keys vs CSR-Based Enrollment
+
+Currently, the server generates the agent's private key during enrollment approval and
+transmits it over the mTLS-secured polling endpoint. This approach was chosen for
+initial implementation simplicity — the agent polls a single endpoint and receives a
+complete PKI bundle without an extra round-trip.
+
+**Mitigations in place:**
+- The PKI bundle is stored in an in-memory cache with single-retrieval semantics —
+  it can only be fetched once and is atomically removed on retrieval.
+- A 10-minute TTL ensures the bundle expires even if never retrieved.
+- The raw polling token is never logged; only its SHA-256 hash is stored.
+
+**Future direction:** A CSR-based enrollment flow should replace server-generated keys.
+Under that model, the agent generates its own key pair locally and submits a Certificate
+Signing Request, eliminating the need for the server to ever hold or transmit the agent's
+private key. This significantly reduces the attack surface.
+
+See: [Issue #9](https://github.com/Draco-Lunaris/Linux-Patch-Manager/issues/9)
+
 ## Credit
 
 Contributors who responsibly report vulnerabilities will be credited in the corresponding GitHub Security Advisory.