nautd: block adding a torrent that would overlap existing data

spawn_torrent parses each torrent's file list (at add and restore) and
refuses an add whose files would write where a registered torrent's data
lives (NAUT_ERR_EXIST). Magnets are checked once metadata is known is
out of scope; restore skips the check. webui surfaces it as HTTP 409.
Mark #10 done.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

include/naut/common.h

@@ -52,6 +52,7 @@ enum {
     NAUT_ERR_FULL  = -8,
     NAUT_ERR_EMPTY = -9,
     NAUT_ERR_NOTFOUND = -10,
+    NAUT_ERR_EXIST    = -11,   /* already exists / data would overlap */
 };
 
 const char *naut_strerror(naut_err e);