Upgrade to drive v0.25.3, replace MinIO with RustFS

- use rustfs instead of minio and provide migration script
- Schedule clean_pending_items/purge_deleted_items with celery beat
- Give celery workers the S3 access key secret
- Collabora admin access needs setting a password

Following the [C4DT AI Policy](https://git.coopcloud.tech/c4dt/ai-policy/src/branch/main/AI-POLICY.md):

 #LLM_vibe
 #LLM_model Claude Opus 5.5

- [x] I take full responsibility for this code
- [x] I understand every line of this code
- [x] I tested this code thoroughly on my own computer / servers
- [x] None of the commit messages were created using LLMs
- [x] I will participate in any discussion on my own and will not use an LLM for the discussion

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

s3-initialize.sh

@@ -0,0 +1,51 @@
+#!/bin/sh
+set -e
+
+# Creates the media bucket on the RustFS object storage, with versioning
+# enabled. Runs inside the backend container, which ships boto3.
+
+# Load secrets into environment
+source /abra-entrypoint.sh -e
+
+python - <<'PYEOF'
+import os
+import sys
+import time
+
+import boto3
+from botocore.exceptions import ClientError, EndpointConnectionError
+
+bucket = os.environ.get("AWS_STORAGE_BUCKET_NAME", "drive-media-storage")
+s3 = boto3.client(
+    "s3",
+    endpoint_url=os.environ["AWS_S3_ENDPOINT_URL"],
+    aws_access_key_id=os.environ["AWS_S3_ACCESS_KEY_ID"],
+    aws_secret_access_key=os.environ["AWS_S3_SECRET_ACCESS_KEY"],
+    region_name=os.environ.get("AWS_S3_REGION_NAME", "us-east-1"),
+)
+
+# Wait for the object storage to be ready (up to 60 seconds)
+for i in range(60):
+    try:
+        s3.list_buckets()
+        break
+    except (ClientError, EndpointConnectionError):
+        time.sleep(1)
+else:
+    sys.exit("s3-initialize: timed out waiting for the object storage")
+
+# Idempotent: skip if bucket already exists
+try:
+    s3.head_bucket(Bucket=bucket)
+    print(f"s3-initialize: bucket '{bucket}' already exists, skipping")
+    sys.exit(0)
+except ClientError:
+    pass
+
+print(f"s3-initialize: creating bucket '{bucket}'...")
+s3.create_bucket(Bucket=bucket)
+s3.put_bucket_versioning(
+    Bucket=bucket, VersioningConfiguration={"Status": "Enabled"}
+)
+print("s3-initialize: done")
+PYEOF