Configure Docker Compose through .env for Arcane

docker-compose.yml now reads every host-specific value (paths, user,
port, network, time zone) from .env with the previous values as
defaults. .env.example documents them; .env stays out of Git.

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

.env.example

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+# playlink settings for docker-compose.yml.
+# Copy to .env (or paste into Arcane's .env editor) and adjust.
+# Anything left out uses the default shown here.
+
+# ---- container ----
+# Image tag to build/run and the container's name.
+PLAYLINK_IMAGE=playlink:latest
+PLAYLINK_CONTAINER_NAME=playlink
+# Version label shown in the UI footer.
+PLAYLINK_VERSION=dev
+# Restart policy: no, always, on-failure or unless-stopped.
+PLAYLINK_RESTART=unless-stopped
+# Existing Docker network to join (must already exist).
+PLAYLINK_NETWORK=chromanet
+
+# ---- user and time zone ----
+# The user and group that own your music files (Unraid nobody:users = 99:100).
+PUID=99
+PGID=100
+TZ=America/Los_Angeles
+
+# ---- web UI ----
+# Host port for the web UI.
+PLAYLINK_PORT=7480
+# Set to true when serving over HTTPS (e.g. behind a reverse proxy).
+PLAYLINK_SECURE_COOKIES=false
+# info, or debug for verbose logs.
+PLAYLINK_LOG_LEVEL=info
+
+# ---- folders ----
+# Host folder for playlink's database and backups (mounted at /config).
+PLAYLINK_CONFIG_PATH=/mnt/user/Aqua/playlink
+# Host folder that contains BOTH libraries (mounted at /data). Hardlinks
+# can't cross bind mounts, so use one common parent folder, e.g.
+#   master (Albums)  -> /data/Media/Music/Albums
+#   slave  (Singles) -> /data/Media/Music/Instances/cdnutter/Singles
+PLAYLINK_DATA_PATH=/mnt/user/Sapphire
+# Folders inside the container that libraries may use (comma-separated).
+PLAYLINK_ALLOWED_ROOTS=/data