Persist settings.json across deploys via a data volume

The previous commit made Settings-saved API keys override .env, but the
key still never survived: settings.json was written to the container
filesystem, and every deploy runs `up -d --build`, which recreates the
container and discards it. Saving a rotated SABnzbd key and restarting
therefore still fell back to the stale .env value.

SETTINGS_FILE is now configurable and pointed at /data/settings.json on
a ./data bind mount, so runtime settings live on the host. The deploy
rsync also excludes data/ — it runs --delete and would otherwise wipe
the directory for not being in the repo, reintroducing the same bug.

This affected everything the Settings page writes, not just API keys:
timezone and a UI-changed login password were being reverted on every
deploy too.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

.forgejo/workflows/deploy.yml

@@ -17,13 +17,15 @@ jobs:
       # at /mnt/user/appdata/greendale on the host, bind-mounted into this
       # container at the same path (see runner container.options) so
       # `docker compose`'s relative paths resolve against the real host
-      # directory rather than an ephemeral one. `.env` is server-only and
-      # must survive every deploy.
+      # directory rather than an ephemeral one. `.env` and `data/` are
+      # server-only state and must survive every deploy — without the
+      # excludes, --delete removes them because they aren't in the repo.
       - name: Sync source to deploy path
         run: |
           rsync -a --delete \
             --exclude='.git' \
             --exclude='.env' \
+            --exclude='data' \
             ./ /mnt/user/appdata/greendale/
 
       - name: Rebuild and restart