feat: M2 — management REST API with OpenAPI doc (control-pane groundwork)

A versioned control-plane REST API (/api/v1) on its own port (default
127.0.0.1:47990) serving host info, runtime status, paired-client
management, the pairing PIN flow, and session control (stop / force-IDR).
The OpenAPI 3.1 document is generated from the handlers by utoipa, served
live at /api/v1/openapi.json (+ Scalar docs at /api/docs), printable via
`lumen-host openapi`, and checked in at docs/api/openapi.json for client
codegen — a test fails if it drifts, mirroring the cbindgen header rule.

Auth: optional bearer token (--mgmt-token / LUMEN_MGMT_TOKEN), enforced on
everything but /health, and mandatory for non-loopback binds. PinGate
gains a waiter count so the API can report pin_pending; logs moved to
stderr so stdout stays machine-readable. Supersedes the web.rs stub.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Cargo.lock

@@ -1493,6 +1493,7 @@ dependencies = [
  "ffmpeg-next",
  "futures-util",
  "hex",
+ "http-body-util",
  "khronos-egl",
  "libc",
  "lumen-core",
@@ -1506,10 +1507,16 @@ dependencies = [
  "rustls",
  "rustls-pemfile",
  "rusty_enet",
+ "serde",
+ "serde_json",
  "sha2",
  "tokio",
+ "tower",
  "tracing",
  "tracing-subscriber",
+ "utoipa",
+ "utoipa-axum",
+ "utoipa-scalar",
  "wayland-backend",
  "wayland-client",
  "wayland-protocols-misc",
@@ -1790,6 +1797,12 @@ dependencies = [
  "windows-link",
 ]
 
+[[package]]
+name = "paste"
+version = "1.0.15"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "57c0d7b74b563b49d38dae00a0c37d4d6de9b432382b2892f0574ddcae73fd0a"
+
 [[package]]
 name = "pem"
 version = "3.0.6"
@@ -3096,6 +3109,55 @@ version = "0.2.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "06abde3611657adf66d383f00b093d7faecc7fa57071cce2578660c9f1010821"
 
+[[package]]
+name = "utoipa"
+version = "5.5.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "8bde15df68e80b16c7d16b9616e80770ad158988daa56a27dccd1e55558b0160"
+dependencies = [
+ "indexmap",
+ "serde",
+ "serde_json",
+ "utoipa-gen",
+]
+
+[[package]]
+name = "utoipa-axum"
+version = "0.2.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "7c25bae5bccc842449ec0c5ddc5cbb6a3a1eaeac4503895dc105a1138f8234a0"
+dependencies = [
+ "axum",
+ "paste",
+ "tower-layer",
+ "tower-service",
+ "utoipa",
+]
+
+[[package]]
+name = "utoipa-gen"
+version = "5.5.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "6ba0b99ee52df3028635d93840c797102da61f8a7bb3cf751032455895b52ef8"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "regex",
+ "syn",
+]
+
+[[package]]
+name = "utoipa-scalar"
+version = "0.3.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "59559e1509172f6b26c1cdbc7247c4ddd1ac6560fe94b584f81ee489b141f719"
+dependencies = [
+ "axum",
+ "serde",
+ "serde_json",
+ "utoipa",
+]
+
 [[package]]
 name = "uuid"
 version = "1.23.2"