feat(M3): Host Management, Groups, Users, CIDR Discovery

- pm-core::models: Host, HostSummary, Group, User, DiscoveryResult
  types + request payloads for all CRUD operations
- pm-core::audit: Tamper-evident hash-chained audit log writer
  (SHA-256 chain, non-fatal, covers all M3 events)
- pm-web/routes/hosts: Full host CRUD with RBAC scoping;
  FQDN DNS resolution on registration; host↔group membership;
  operator group-scoped access enforcement; audit on register/remove
- pm-web/routes/groups: Full group CRUD; host↔group and user↔group
  membership management; admin-only create/delete/update
- pm-web/routes/users: Full user CRUD (admin); current user profile;
  password hashing (Argon2id); role management; session revocation
- pm-web/routes/discovery: CIDR scan with bounded concurrency
  (128 workers), TCP probe with 2s timeout, reverse DNS lookup,
  scan results table, register-from-discovery flow with audit log
- Frontend: HostsPage (filterable table with health chips),
  HostDetailPage, GroupsPage (create/delete dialog),
  UsersPage (create/revoke sessions)
- App.tsx updated with all M3 routes wired to real pages
- cargo check --workspace: zero errors

Closes M3.

crates/pm-core/Cargo.toml

@@ -20,3 +20,5 @@ ulid = { workspace = true }
 chrono = { workspace = true }
 config = { workspace = true }
 axum = { workspace = true }
+sha2 = { workspace = true }
+hex = { workspace = true }