Add ability for admin operators to take over user conversation sessions
from the AI bot, manually reply as the bot, and release sessions back to
AI processing.
Backend:
- New HumanTakeoverSession DB model and migration (v26)
- HumanTakeoverService with in-memory cache for hot-path performance
- REST API endpoints for takeover/release/send-message/list/detail
- Message interception in botmgr.py (after webhook, before pipeline)
Frontend:
- Takeover/release controls in BotSessionMonitor chat header
- Operator message input bar with visual distinction (orange theme)
- Taken-over session indicators in session list
- 3-second auto-refresh polling during active takeover
- Full i18n coverage across all 7 locales
