Librarian: answer each query back to the bot that sent it

So one librarian can serve several bots (test + deploy) instead of firing
every result/pong at a single static CONJURER_MAIN_BOT.

* The bot includes its own callback address (CONJURER_SELF_CALLBACK) in
  every /query and /ping.
* The librarian stores that callback with the query (persisted with the
  request, so a replay after restart still answers the right bot) and, for
  results, in the OUTBOX entry ({target, payload}) so the resender delivers
  to the origin bot even across a librarian restart.
* Pongs go back to the pinging bot too - otherwise a second bot's health
  check would be ponged to the first and always time out, so it could
  never enable its librarian cog.
* Empty callback falls back to MAIN_BOT_ADDRESS, and a legacy OUTBOX entry
  (raw payload, pre-callback) is still delivered to the default bot, so the
  upgrade is seamless.

Tests: per-origin result delivery + legacy-shape fallback (outbox),
busy/idle pong routed to the callback bot vs default (lifecycle). Suite:
58 unit + 52 integration green.

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

constants.py

@@ -254,6 +254,10 @@ SKIP_TRACK = os.getenv("CONJURER_SKIP_ENDPOINT", "/skip")
 LIBRARIAN_SERVICE_ADDRESS = os.getenv(
     "CONJURER_LIBRARIAN_SERVICE", "http://192.168.1.192:5001"
 )
+# The address the librarian (and its pongs) should send results BACK to for THIS
+# bot - so one librarian can serve several bots (test + deploy), each getting its
+# own answers. Empty => the librarian falls back to its static CONJURER_MAIN_BOT.
+SELF_CALLBACK = os.getenv("CONJURER_SELF_CALLBACK", "")
 HOST_ADDRESS = os.getenv("CONJURER_DISCORD_HOST", "192.168.1.191")
 PORT_ADDRESS = int(os.getenv("CONJURER_DISCORD_PORT", "5000"))