downloader: fix double-prefixed webseed metainfo URL (#19200)

## Summary

- PR #18755 ("Add webseed checker") accidentally passed `base` for both
`webseedUrlBase` and `name` in `GetMetainfoFromWebseed`, producing
doubled URLs like `https://host/https://host/.torrent` which return HTTP
404
- Fix: pass the snapshot `name` instead of `base` as the second argument

Fixes #19094

## Test plan

- [x] Built and ran an ephemeral mainnet instance for 2 minutes — no
`"error fetching metainfo from webseeds"` warnings, all 305/305 file
metadata fetched, webseed download steady at ~62-70 MB/s

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: wmitsuda

db/downloader/downloader.go

@@ -1074,7 +1074,7 @@ func (d *Downloader) fetchMetainfoFromWebseeds(ctx context.Context, name string,
 		buf.Reset()
 		var mi metainfo.MetaInfo
 		var w io.Writer = &buf
-		mi, err = GetMetainfoFromWebseed(ctx, base, base, d.metainfoHttpClient, w)
+		mi, err = GetMetainfoFromWebseed(ctx, base, name, d.metainfoHttpClient, w)
 		if err != nil {
 			d.log(log.LvlDebug, "error fetching metainfo from webseed", "err", err, "name", name, "webseed", base)
 			// Whither error?