Rename host library to master throughout

The two libraries are now "master" and "slave" everywhere: UI text, API
JSON fields (master_root, rel_master, master_size, ...), Go identifiers,
database columns, quality verdicts, quarantine subfolders and docs.

Existing databases are upgraded by a new migration that renames the
columns in place and rewrites stored "host" values, so pairs, history,
cached verdicts and restorable quarantine entries carry over. Network
"host" usages (SplitHostPort, the Origin/Host check) are unchanged.

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

internal/fsops/fsops.go

@@ -23,7 +23,7 @@ import (
 )
 
 // TmpPrefix marks staging files created by playlink. Anything carrying this
-// prefix is disposable: it is either a hardlink to a host file or a copy that
+// prefix is disposable: it is either a hardlink to a master file or a copy that
 // has not been renamed into place yet.
 const TmpPrefix = ".playlink-tmp-"