feat(M1): Project scaffolding, DB schema, core infrastructure

- Initialize Rust workspace with 7 crates (pm-web, pm-worker, pm-core,
  pm-agent-client, pm-auth, pm-ca, pm-reports)
- React + TypeScript + Vite + MUI frontend scaffold
- Full PostgreSQL schema: all 17 tables with indexes and constraints
- pm-core: config (TOML+env), db (SQLx pool + migrations), error
  (unified AppError + JSON envelope), request_id (ULID middleware),
  logging (tracing JSON/pretty)
- pm-web: Axum skeleton, /status/health endpoint, static file serving
- pm-worker: Tokio skeleton, heartbeat writer, schema version check
- Embedded sqlx migrations with advisory lock (single-writer)
- systemd unit files, setup.sh, build-frontend.sh
- config.example.toml with all configuration keys
- docs/runbooks/restore.md
- cargo check passes with zero warnings

Closes M1.

crates/pm-agent-client/src/client.rs

@@ -0,0 +1,10 @@
+//! Agent HTTP client stub.
+//! Full mTLS Rustls-based implementation arrives in M4.
+
+use thiserror::Error;
+
+#[derive(Debug, Error)]
+pub enum AgentClientError {
+    #[error("Not yet implemented")]
+    NotImplemented,
+}

crates/pm-agent-client/src/lib.rs

@@ -0,0 +1,4 @@
+//! pm-agent-client — mTLS HTTP client for Linux Patch API agent communication.
+//!
+//! M1: Stub. Full implementation in M4.
+pub mod client;