Add automated daily Gitea backup

Alpine sidecar container runs gitea dump via docker exec at 2 AM daily,
writing backups to /data/backup/gitea/ with 7-day retention.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backup.sh

@@ -0,0 +1,57 @@
+#!/bin/sh
+set -e
+
+BACKUP_DIR="/backup"
+RETAIN_DAYS="${BACKUP_RETAIN_DAYS:-7}"
+TIMESTAMP="$(date +%Y-%m-%d_%H%M)"
+BACKUP_FILE="gitea-dump-${TIMESTAMP}.zip"
+
+echo "========================================"
+echo "Gitea backup started at $(date)"
+echo "========================================"
+
+# Pre-flight check
+if ! docker inspect gitea >/dev/null 2>&1; then
+    echo "ERROR: gitea container not found or not running."
+    exit 1
+fi
+
+# Ensure backup directory exists inside gitea container
+docker exec -u git gitea sh -c "mkdir -p /tmp/backup"
+
+# Run gitea dump
+echo "Running gitea dump..."
+docker exec -u git -w /tmp gitea \
+    /app/gitea/gitea dump \
+    -c /data/gitea/conf/app.ini \
+    --file "/tmp/backup/${BACKUP_FILE}" \
+    --skip-log \
+    --type zip
+
+# Verify backup was created
+if [ ! -f "${BACKUP_DIR}/${BACKUP_FILE}" ]; then
+    echo "ERROR: Backup file not found at ${BACKUP_DIR}/${BACKUP_FILE}"
+    exit 1
+fi
+
+FILESIZE=$(stat -c%s "${BACKUP_DIR}/${BACKUP_FILE}" 2>/dev/null || stat -f%z "${BACKUP_DIR}/${BACKUP_FILE}" 2>/dev/null)
+echo "Backup created: ${BACKUP_FILE} (${FILESIZE} bytes)"
+
+# Retention: delete backups older than N days
+echo "Cleaning up backups older than ${RETAIN_DAYS} days..."
+DELETED=$(find "${BACKUP_DIR}" -name "gitea-dump-*.zip" -mtime "+${RETAIN_DAYS}" -print -delete)
+if [ -n "${DELETED}" ]; then
+    echo "Deleted old backups:"
+    echo "${DELETED}"
+else
+    echo "No old backups to delete."
+fi
+
+echo ""
+echo "Current backups:"
+ls -lh "${BACKUP_DIR}"/gitea-dump-*.zip 2>/dev/null || echo "(none)"
+
+echo ""
+echo "========================================"
+echo "Gitea backup completed at $(date)"
+echo "========================================"