Add Forgejo deploy workflow and update deploy docs

.forgejo/workflows/deploy.yml

@@ -0,0 +1,28 @@
+name: Deploy
+
+on:
+  push:
+    branches: [main]
+
+jobs:
+  deploy:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      # The job container only sees its own checkout — the live app lives
+      # at /mnt/user/appdata/card-grader on the host, bind-mounted into
+      # this container at the same path (see runner container.options) so
+      # `docker compose`'s relative volume paths resolve against the real
+      # host directory rather than an ephemeral one. `data/` holds the
+      # production DB and photos and must survive every deploy.
+      - name: Sync source to deploy path
+        run: |
+          rsync -a --delete \
+            --exclude='.git' \
+            --exclude='data' \
+            ./ /mnt/user/appdata/card-grader/
+
+      - name: Rebuild and restart
+        working-directory: /mnt/user/appdata/card-grader
+        run: docker compose up -d --build