fix: route V1 ListObjects to the list handler instead of raw-forwarding

[ServerSideHannes]

Problem

Even after #92 (serve V1 ListObjects via the backend's V2 API), Scylla backups and Postgres retention still got HTTP 400 from Hetzner. Reason: the request never reached the fixed handler.

_dispatch_bucket raw-forwards any bucket GET whose query lacks list-type/delete/uploads/location:

skip_queries = (QUERY_LIST_TYPE, QUERY_DELETE, QUERY_UPLOADS, QUERY_LOCATION)
if query and not any(q in query for q in skip_queries):
    return await self.handler.forward_request(request, creds)   # raw passthrough

A V1 ListObjects is GET /bucket?prefix&delimiter&max-keys&encoding-type with no list-type=2 → it matched this catch-all and was forwarded verbatim to the backend as V1 → Hetzner 400. handle_list_objects_v1 was only reached for a bare GET /bucket (empty query), so #92's translation never applied to what rclone / barman actually send.

Confirmed on 2026.6.9: backend request logged as GET https://hel1.your-objectstorage.com/…-scylladb-backups-v3?delimiter=%2F&encoding-type=url&max-keys=1000&prefix=… → HTTP/2 400 (no list-type=2 → still V1).

Fix

Treat a bucket GET whose query is only listing params (prefix, delimiter, marker, max-keys, encoding-type, fetch-owner, start-after, continuation-token, list-type) as a listing → fall through to the list handler (which serves V1 via the backend's V2 API). Genuine sub-resource GETs (acl, versioning, cors, location, …) still raw-forward unchanged.

Test

tests/unit/test_dispatch_v1_list.py: V1-with-params and bare GET /bucket route to handle_list_objects_v1; list-type=2 → V2 handler; ?acl/?versioning still forward; ?location → bucket-location. Pre-fix the V1-with-params case routes to forward (asserted). Full unit suite green (546 passed); ruff check + format clean.

Completes the chain: #91 (V2 token) → #92 (V1→V2 in handler) → this (route V1 to the handler).