Serve stale virtual members while they refresh (#167)

Virtual merges expire after 60s and the next request blocks on every member. One slow member (a large github_rpm remote) makes the re-merge outlast dnf's 30s timeout.

- add `serve_stale` to remotes (model, v2 API, migration 0008, UI, TUI), default false
- serve a flagged member's last good fetch to rpm/deb/alpine merges at once
- refresh it in the background, single-flighted, detached from the request
- block on the member again once its copy is over an hour old, or the flag is off
- prune copies with their merge cache; drop refreshes for evicted entries

Reviewed-on: #167
Co-authored-by: unkin-agent <unkin-agent@unkin.net>
Co-committed-by: unkin-agent <unkin-agent@unkin.net>

ui/src/api/types.ts

@@ -17,6 +17,7 @@ export interface Remote {
   quarantine_enabled: boolean;
   quarantine_days: number;
   stale_on_error: boolean;
+  serve_stale: boolean;
   releases_remote: string;
   managed_by: string;
   created_at: string;

ui/src/pages/RemoteDetail.tsx

@@ -50,6 +50,8 @@ export function RemoteDetail() {
             <dd>{remote.check_mutable ? 'enabled' : 'disabled'}</dd>
             <dt>Stale on Error</dt>
             <dd>{remote.stale_on_error ? 'enabled' : 'disabled'}</dd>
+            <dt>Serve Stale</dt>
+            <dd>{remote.serve_stale ? 'enabled' : 'disabled'}</dd>
             {remote.releases_remote && (
               <>
                 <dt>Releases Remote</dt>