Dockerize chat_backend for server-infra deploy (closes #6).

Add uv packaging, env-driven Django settings, OLLAMA_BASE_URL (LAN GPU
host), DatabaseStorage for prompt/document blobs, compose/CI/deploy
workflows mirroring scha, and ASGI gunicorn+UvicornWorker for WebSockets.

scripts/validate-env.sh

@@ -0,0 +1,52 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+ENV_FILE="${1:?Usage: $0 <env-file>}"
+
+if [[ ! -f "$ENV_FILE" ]]; then
+  echo "Environment file not found: $ENV_FILE" >&2
+  exit 1
+fi
+
+set -a
+# shellcheck disable=SC1090
+source "$ENV_FILE"
+set +a
+
+DJANGO_ENV="${DJANGO_ENV:-prod}"
+
+required_vars=(
+  DJANGO_SECRET_KEY
+  DJANGO_ALLOWED_HOSTS
+  DATABASE_URL
+  WEB_PORT
+  OLLAMA_BASE_URL
+)
+
+if [[ "$DJANGO_ENV" == "prod" || "$DJANGO_ENV" == "beta" ]]; then
+  required_vars+=(
+    EMAIL_HOST_USER
+    EMAIL_HOST_PASSWORD
+  )
+fi
+
+missing=()
+for var in "${required_vars[@]}"; do
+  if [[ -z "${!var:-}" ]]; then
+    missing+=("$var")
+  fi
+done
+
+if ((${#missing[@]} > 0)); then
+  echo "Missing required environment variables in $ENV_FILE:" >&2
+  printf '  - %s\n' "${missing[@]}" >&2
+  echo "Copy .env.prod.example to ~/Documents/secrets/chat_backend/chat_backend_${DJANGO_ENV}.env and set production values." >&2
+  exit 1
+fi
+
+if [[ "$DJANGO_ENV" == "prod" && ( "$DJANGO_SECRET_KEY" == change-me* || "$DJANGO_SECRET_KEY" == *dev-only* ) ]]; then
+  echo "DJANGO_SECRET_KEY must be changed from the example value for production." >&2
+  exit 1
+fi
+
+echo "Environment validation passed (DJANGO_ENV=$DJANGO_ENV)."