Fix #196: one-command operator backup (pg_dump + MinIO)

Move backup from a documented procedure to `deploy/backup.sh`: dumps Postgres
(pg_dump --clean --if-exists, gzipped) and archives the MinIO /data directory
into a single timestamped bundle under backups/. Reads config from the compose
.env with the same defaults the stack uses; optional BACKUP_RETAIN_DAYS prunes
old bundles (cron-friendly). BACKUP.md documents usage + the restore procedure
(kept manual/documented rather than an untested destructive script).

Closes #196

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Signed-off-by: Justin Paul <justin@jpaul.me>

deploy/BACKUP.md

@@ -0,0 +1,55 @@
+# Backup & restore
+
+`backup.sh` produces a single bundle containing the Postgres database and the
+MinIO object store. Run it from this `deploy/` directory on the host that runs
+the stack.
+
+## Back up
+
+```bash
+./backup.sh
+# → backups/provenance-backup-20260609T140000Z.tar
+```
+
+The bundle contains:
+
+- `db.sql.gz` — `pg_dump --clean --if-exists` of the database, gzipped.
+- `minio-data.tar.gz` — the MinIO `/data` directory (objects + bucket metadata).
+- `MANIFEST.txt` — what's inside and when it was made.
+
+Optional retention: `BACKUP_RETAIN_DAYS=30 ./backup.sh` also deletes bundles
+older than 30 days. Schedule it from cron for off-box copies, e.g.:
+
+```cron
+15 3 * * *  cd /path/to/provenance/deploy && BACKUP_RETAIN_DAYS=30 ./backup.sh
+```
+
+(Copy the resulting bundle off the host — a backup on the same disk isn't one.)
+
+## Restore
+
+Restoring overwrites live data — stop the app first.
+
+```bash
+ts=20260609T140000Z                      # the bundle you're restoring
+mkdir -p /tmp/restore && tar xf backups/provenance-backup-$ts.tar -C /tmp/restore
+
+# 1. Database — the dump is --clean, so it drops & recreates objects.
+docker compose stop backend worker
+gunzip -c /tmp/restore/db.sql.gz \
+  | docker compose exec -T postgres psql -U "${POSTGRES_USER:-provenance}" -d "${POSTGRES_DB:-provenance}"
+
+# 2. Objects — replace the MinIO data directory.
+docker compose stop minio
+docker compose run --rm --no-deps -T -v provenance_miniodata:/data minio \
+  sh -c 'rm -rf /data/* && tar xzf - -C /data' < /tmp/restore/minio-data.tar.gz
+docker compose up -d
+
+rm -rf /tmp/restore
+```
+
+Notes:
+- The MinIO `/data` archive is filesystem-level; restore into the **same** MinIO
+  major version it was taken from.
+- Verify the volume name (`docker volume ls | grep miniodata`) — compose prefixes
+  it with the project name; adjust the `-v` mount accordingly.