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>

docker-compose.yml

@@ -16,7 +16,7 @@ services:
       - /mnt/user/Aqua/playlink:/config
       # IMPORTANT: mount ONE folder that contains BOTH libraries. Hardlinks
       # cannot cross Docker bind mounts, even on the same disk.
-      #   host  (Albums)  -> /data/Music/Albums
+      #   master  (Albums)  -> /data/Music/Albums
       #   slave (Singles) -> /data/Music/Instances/cdnutter/Singles
       - /mnt/user/Sapphire:/data
     restart: unless-stopped