build: 打通 FastAPI 与 Vue 最小工程、依赖与数据库连接

后端入口、配置、Redis 与 Alembic 迁移就位；前端 Vite 工程可安装可构建。

backend/alembic/versions/add_allow_new_chat_001.py

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+"""
+Copyright 2024-2026 ChatterMate
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+"""
+
+"""add agent_customizations.allow_new_chat
+
+Revision ID: add_allow_new_chat_001
+Revises: repair_agent_role_seed_001
+Create Date: 2026-08-09
+
+Shows a "New chat" control in the widget so a visitor can abandon the current
+conversation and start a clean one. Defaults false: starting a new chat closes
+the session, which would cut off a human agent mid-handover, so operators opt in.
+"""
+from alembic import op
+import sqlalchemy as sa
+
+# revision identifiers, used by Alembic.
+revision = 'add_allow_new_chat_001'
+down_revision = 'repair_agent_role_seed_001'
+branch_labels = None
+depends_on = None
+
+
+def _has_column() -> bool:
+    """Development databases are shared between feature branches, so the column can
+    already exist by the time this runs. Adding it twice would abort the upgrade."""
+    inspector = sa.inspect(op.get_bind())
+    return any(c['name'] == 'allow_new_chat'
+               for c in inspector.get_columns('agent_customizations'))
+
+
+def upgrade() -> None:
+    if _has_column():
+        return
+    op.add_column(
+        'agent_customizations',
+        sa.Column('allow_new_chat', sa.Boolean(),
+                  server_default=sa.text('false'), nullable=False),
+    )
+
+
+def downgrade() -> None:
+    if _has_column():
+        op.drop_column('agent_customizations', 'allow_new_chat')