fix(quota): admit writes against a persisted usage baseline while authoritative usage is unavailable (#5722)

Upgrading from a pre-v2 release leaves only the legacy .usage.json snapshot, which has no completeness marker and is demoted to non-authoritative, so every write to a quota-enabled bucket failed closed with a retryable 503 until the scanner's first complete cycle persisted .usage.v2.json — a production outage on large namespaces (issue #5716).

Quota admission now degrades to the last persisted per-bucket size: normalize_loaded_data_usage returns the pre-discard bucket sizes, the TTL-bounded snapshot cache retains them (carried forward through failed refreshes), and QuotaChecker::get_real_time_usage falls back to that baseline when the authoritative caches miss. The baseline is static between snapshot loads, so hard-quota enforcement is advisory for the duration of the degraded window — strictly tighter than beta.11 (usage treated as 0) and strictly more available than a blanket 503. Buckets absent from every persisted snapshot still fail closed, and removing a bucket's usage from the backend purges the baseline so a recreated bucket cannot inherit the dead incarnation's size.

Refs #5716

docs/architecture/compat-cleanup-register.md

@@ -17,7 +17,7 @@ for later deletion.
 - `rustfs-5416-wait-mode` startup wait-mode validation: releases before explicit local endpoint identity treat an unknown RUSTFS_STARTUP_TOPOLOGY_WAIT_MODE value as auto. New servers retain that fallback only when no explicit local endpoint host is configured; an anchor requires a recognized mode so a typo cannot bypass DNS locality. Remove the fallback and reject every unknown value after every supported direct-upgrade chart validates this setting before rollout.
 - `rustfs-5416-kubernetes-alias-dns` Kubernetes endpoint identity fallback: deployments created before explicit local endpoint identity may use resolvable aliases that do not match the Pod hostname. An implicit auto-mode zero match retains legacy DNS locality with a bounded deadline, while ambiguous matches and invalid explicit anchors still fail closed. Remove the fallback after every supported direct-upgrade chart and deployment manifest provides a canonical RUSTFS_LOCAL_ENDPOINT_HOST for domain-based distributed topologies.
 - `rustfs-5416-zero-retry-delay` startup retry-delay validation: releases before bounded topology convergence accept RUSTFS_STARTUP_TOPOLOGY_RETRY_MAX_DELAY values of 0 or 0ms. New servers replace those values with the safe nonzero default so a direct upgrade neither fails startup nor enters a busy loop. Reject zero after the minimum supported direct-upgrade release validates or rewrites this setting before rollout.
-- `scanner-usage-v2` persisted scanner usage migration: pre-v2 scanners write `.usage.json`, so upgraded clusters read that primary/backup pair only while `.usage.v2.json` is absent and continue removing deleted buckets from legacy copies that still exist. The additive usage_snapshot_complete field in `.usage.v2.json` must remain optional while mixed-version clusters are supported; a missing field means the snapshot is not authoritative. Remove the legacy object fallback and cleanup only after every supported direct-upgrade source writes `.usage.v2.json`.
+- `scanner-usage-v2` persisted scanner usage migration: pre-v2 scanners write `.usage.json`, so upgraded clusters read that primary/backup pair only while `.usage.v2.json` is absent and continue removing deleted buckets from legacy copies that still exist. The additive usage_snapshot_complete field in `.usage.v2.json` must remain optional while mixed-version clusters are supported; a missing field means the snapshot is not authoritative. The legacy read also feeds the degraded quota-admission baseline (issue #5716): while no authoritative usage exists, quota checks admit against the pre-discard sizes of the last loaded snapshot, including a legacy one. Remove the legacy object fallback and cleanup only after every supported direct-upgrade source writes `.usage.v2.json`; the baseline then feeds from incomplete v2 snapshots alone.
 - `ns-scanner-rpc-v3` namespace scanner capability and activity handshake: old peers and legacy internode transports lack the authenticated startup-epoch handshake. The oldest peers send an empty activity request and receive a field-empty protocol-0 response. Protocol v4 binds the challenge and response topology but cannot authenticate distributed dirty-usage state. Protocol v5 binds the request version, acknowledgement target and generation, and the response dirty-usage state, but predates set-scoped scanner cache locks. Current protocol v6 additionally fences scanner cache lock-domain changes, so distributed scanner cycles publish usage only after every peer reports protocol v6 state. Servers retain protocol-0 and protocol-v4 codecs for rolling upgrades, while protocol-v5 peers are treated as previous-version peers that cannot safely participate in the new cache lock domain. Scanner selection treats HTTP 404/405/426 and the legacy MethodNotAllowed default as an explicit lack of remote scanner v3 support and assigns those disks to coordinator-driven workers; transient capability failures remain incomplete and do not activate the fallback. Remove the coordinator fallback after the minimum supported RustFS peer version implements namespace scanner protocol v3, remove protocol-0 activity requests and responses after every supported peer implements authenticated scanner activity protocol v4, remove the protocol-v4 activity codec after every supported peer implements protocol v5, and remove protocol-v5 previous-version rejection after every supported peer implements protocol v6; future protocol revisions must keep the same dual-version server/codec window before changing the advertised version.
 - `#4648` walk-dir stream completion capability: old clients can append fallback output to an already-used metacache writer after a terminal body error, so servers emit terminal walk errors only to clients that sign the `walk_dir_stream_completion=error-v1` query capability and its request-body digest. Remove the legacy clean-EOF path after the minimum supported RustFS peer version always advertises this capability.
 - `heal-rpc-auth-v2` internode gRPC authentication: servers temporarily accept legacy prefix signatures so old peers remain available during rolling upgrades. Remove the legacy fallback after the minimum supported RustFS peer version sends v2 authentication on every internode gRPC request.