docker: Proxmox deployment for bot/librarian/musician + code fixes

Containerises the three services (each intended for its own Proxmox VM)
and adds the code changes needed to run cleanly on Linux/Docker.

Code fixes:
- constants.py: CONJURER_DATA_DIR roots all writable bot state under one
  mounted volume (per-variable overrides still win; native Pi unaffected)
- conjurer_librarian/search_bot.py + scrape_bot.py: the hardcoded Windows
  DOI database path (C:\Database\chunks\) is now CONJURER_LIBRARIAN_DB_PATH,
  with CONJURER_LIBRARIAN_MAXTHREADS / _CHUNK also env-overridable

Docker:
- docker/Dockerfile.{bot,librarian,musician} + compose.{bot,librarian,musician}.yaml
- docker/env/*.env.example (force-added; real *.env stays gitignored)
- docker/entrypoint.bot.sh seeds default JSON state into /data only when
  absent, so preserved history is never overwritten
- .dockerignore
- docs/deployment/DOCKER_PROXMOX.md: step-by-step runbook incl. preserving
  the existing command/conversation history and cross-VM auth

The bot image uses the vendored yt_dlp/spotify_dl forks (they win on
sys.path over the pip packages), dropping the old sed patching.

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

docker/entrypoint.bot.sh

@@ -0,0 +1,16 @@
+#!/bin/sh
+# Seed the data volume with the baked-in default JSON state on first run only.
+# Existing files (e.g. your preserved pamiec.json history) are never overwritten.
+set -e
+
+DATA="${CONJURER_DATA_DIR:-/data}"
+mkdir -p "$DATA"
+
+for f in settings.json system_gpt_settings.json pamiec.json pamiec_muzyki.json accident_log.json; do
+    if [ ! -e "$DATA/$f" ] && [ -e "/app/$f" ]; then
+        cp "/app/$f" "$DATA/$f"
+        echo "entrypoint: seeded $f into $DATA"
+    fi
+done
+
+exec "$@"