docs: document LOCAL_BIND_URL (constitution IV)

#145 added the optional LOCAL_BIND_URL (the secondary loopback listener,
default localhost:8088, empty = skip) but documented it only in
CLAUDE.md. Constitution IV requires every new environment variable in
the README environment section; also add it to .env.example.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.env.example

@@ -11,6 +11,9 @@ VIDEO_PATH=/path/to/video/hls
 GIFS_DIRECTORY=/path/to/gifs
 PREVIEW_CLIPS_DIRECTORY=/path/to/preview-clips
 BIND_URL=0.0.0.0:8080
+# Secondary loopback listener (default localhost:8088). Empty = don't bind it
+# (e.g. a second instance next to one that already owns 8088).
+# LOCAL_BIND_URL=localhost:8088
 CORS_ALLOWED_ORIGINS=http://localhost:3000
 SECRET_KEY=replace-me-with-a-long-random-secret
 RUST_LOG=info

README.md

@@ -71,6 +71,7 @@ They should be defined where the binary is located or above it in an `.env` file
 - `VIDEO_PATH` is a path where HLS playlists and video parts should be stored
 - `GIFS_DIRECTORY` is a path where generated video GIF thumbnails should be stored
 - `BIND_URL` is the url and port to bind to (typically your own IP address)
+- `LOCAL_BIND_URL` (optional) is a second, loopback listener [default: `localhost:8088`]. Set it to an empty string to skip it, e.g. to run a second instance such as the `dev/e2e` harness alongside one that already owns 8088
 - `SECRET_KEY` is the *hopefully* random string to sign Tokens with
 - `RUST_LOG` is one of `off, error, warn, info, debug, trace`, from least to most noisy [default: `info` for spans, `error` for dev console logs]. It also gates tracing-span export, so keep it at `info` or `debug`
 - `OTLP_OTLS_ENDPOINT` (optional) is an OTLP/gRPC collector URL (e.g. SigNoz, Jaeger) for span export, in any build [default: unset, no export]