Add streaming brotli compression and hot-reloadable config (livevue.toml)

Key changes:

- **src/config.rs** – `LiveVueConfig` (server host/port, brotli quality +
  window size), loaded from `livevue.toml`. A notify-based file watcher
  reloads the config on any file change; a SIGHUP handler does the same on
  demand. Falls back to compiled-in defaults when the file is absent.

- **src/brotli_layer.rs** – `BrotliBody`, a custom `http_body::Body` wrapper
  that streams brotli-compressed data through a single persistent
  `brotli::CompressorWriter`. The encoder survives across SSE event flushes,
  so its sliding window accumulates context from all prior events —
  progressively better compression as the stream grows. Both quality (0–11)
  and window size (lgwin 10–24, i.e. 1 KB – 16 MB) are taken from config.
  `brotli_compression` is an axum `from_fn_with_state` middleware that
  activates only when the client sends `Accept-Encoding: br`.

- **src/server.rs** – `AppState` gains a `config: SharedConfig` field.
  `AppState::new` takes the config; `AppState::new_default` is a zero-config
  convenience constructor.

- **livevue.toml** – documented example config with inline comments
  explaining each field and the brotli window-size tradeoff table.

- **examples/todo** – loads config via `load_and_watch_config`, derives the
  bind address from `config.server`, and applies `brotli_compression` as a
  router layer.

https://claude.ai/code/session_01UnQQkkwts64FPUsSzFfdQb

livevue.toml

@@ -0,0 +1,68 @@
+# livevue.toml — livevue-rs server configuration
+#
+# This file is optional. The server starts with compiled-in defaults when it
+# is absent. When present, it is loaded on startup and then watched for
+# changes automatically (inotify on Linux, kqueue on macOS, FSEvents on macOS).
+#
+# To force a reload without restarting: kill -HUP <pid>
+#
+# Note: [server] changes (host / port) require a full restart because the TCP
+# listener is bound once at startup. All brotli settings apply to new
+# connections immediately after a reload.
+
+# ---------------------------------------------------------------------------
+# HTTP server
+# ---------------------------------------------------------------------------
+
+[server]
+
+# Address to bind. "0.0.0.0" listens on all interfaces.
+host = "0.0.0.0"
+
+# TCP port.
+port = 3000
+
+# ---------------------------------------------------------------------------
+# Brotli streaming compression
+# ---------------------------------------------------------------------------
+#
+# Brotli is the compression algorithm of choice for Datastar SSE streams.
+# Unlike gzip (which compresses each HTTP response independently), brotli
+# maintains a *sliding window* across the entire SSE byte stream. All events
+# on a connection share a single encoder context, so later events — which
+# often contain similar HTML structure — get compressed with reference to
+# everything that was sent earlier. Compression ratios improve progressively
+# over the lifetime of a long-lived connection.
+#
+# The browser sends "Accept-Encoding: br" if it supports brotli. The
+# middleware checks this and only activates compression for those clients.
+
+[brotli]
+
+# Set to false to disable brotli and send SSE events uncompressed.
+enabled = true
+
+# Encoder quality: 0 (fastest, worst ratio) … 11 (slowest, best ratio).
+#
+# For real-time SSE, 4–6 gives a good latency/ratio tradeoff. Values above 9
+# incur significant CPU cost with diminishing returns for typical HTML
+# payloads. Default: 5.
+quality = 5
+
+# Sliding window size as log₂ of bytes (brotli's `lgwin` parameter).
+#
+# The window is how much previously compressed data the encoder can
+# reference. Larger ⇒ better compression, more memory per connection.
+#
+#   lgwin │ Window size
+#   ──────┼────────────
+#     10  │ 1 KB
+#     16  │ 64 KB
+#     20  │ 1 MB
+#     22  │ 4 MB   ← default
+#     24  │ 16 MB
+#
+# For a server with many concurrent SSE connections, consider whether the
+# memory cost (window_size bytes × number of connections) is acceptable.
+# Reduce to 20 or 16 under memory pressure.
+window_size = 22