feat(human-takeover): add human customer service takeover feature

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

src/langbot/pkg/entity/persistence/human_takeover.py

@@ -0,0 +1,36 @@
+import sqlalchemy
+
+from .base import Base
+
+
+class HumanTakeoverSession(Base):
+    """Human takeover session records.
+
+    Tracks which conversation sessions are currently under human operator control,
+    bypassing the normal AI pipeline processing.
+    """
+
+    __tablename__ = 'human_takeover_sessions'
+
+    id = sqlalchemy.Column(sqlalchemy.String(255), primary_key=True)
+    session_id = sqlalchemy.Column(sqlalchemy.String(255), nullable=False, unique=True, index=True)
+    """Corresponds to monitoring_sessions.session_id, format: 'person_{id}' or 'group_{id}'"""
+
+    bot_uuid = sqlalchemy.Column(sqlalchemy.String(255), nullable=False, index=True)
+    """UUID of the bot whose session is being taken over"""
+
+    status = sqlalchemy.Column(sqlalchemy.String(50), nullable=False, default='active', index=True)
+    """Takeover status: 'active' or 'released'"""
+
+    taken_by = sqlalchemy.Column(sqlalchemy.String(255), nullable=True)
+    """Email/username of the admin who took over the session"""
+
+    taken_at = sqlalchemy.Column(sqlalchemy.DateTime, nullable=False)
+    """Timestamp when the takeover started"""
+
+    released_at = sqlalchemy.Column(sqlalchemy.DateTime, nullable=True)
+    """Timestamp when the takeover was released (null if still active)"""
+
+    platform = sqlalchemy.Column(sqlalchemy.String(255), nullable=True)
+    user_id = sqlalchemy.Column(sqlalchemy.String(255), nullable=True)
+    user_name = sqlalchemy.Column(sqlalchemy.String(255), nullable=True)