feat: chat language display, disabled EntitySelect items, dev scripts

Chat language:
- Added language_code field to TelegramChat model + migration
- Saved from message.from.language_code on webhook/polling
- Displayed as badge on bot chat cards and target receiver items
- Resolved from DB in target API response (works for existing receivers)
- Shown in chat picker dropdown (desc includes language)

EntitySelect improvements:
- Tracker-target link selector shows all targets, already-linked ones
  appear disabled with "Already linked" hint
- Receiver chat picker shows already-added chats as disabled

Dev scripts:
- scripts/restart-backend.sh and restart-frontend.sh
- Updated .claude/docs/dev-servers.md to reference scripts

scripts/restart-backend.sh

@@ -0,0 +1,22 @@
+#!/usr/bin/env bash
+# Restart the backend dev server.
+# Usage: bash scripts/restart-backend.sh
+
+set -e
+cd "$(dirname "$0")/.."
+
+# Kill existing backend
+PID=$(netstat -ano 2>/dev/null | grep ':8420.*LISTENING' | awk '{print $5}' | head -1)
+if [ -n "$PID" ]; then
+  taskkill //F //PID "$PID" 2>/dev/null || true
+  sleep 1
+fi
+
+# Start backend
+NOTIFY_BRIDGE_DATA_DIR=./test-data \
+NOTIFY_BRIDGE_SECRET_KEY=test-secret-key-minimum-32chars \
+nohup python -m uvicorn notify_bridge_server.main:app \
+  --host 0.0.0.0 --port 8420 > /dev/null 2>&1 &
+
+sleep 3
+curl -s http://localhost:8420/api/health