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_chat_session_idx_001.py

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+"""
+Copyright 2024-2026 ReplyPier
+
+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.
+"""
+
+"""index chat_history on (session_id, created_at)
+
+Revision ID: add_chat_session_idx_001
+Revises: hc_img_paths_001
+Create Date: 2026-07-22
+
+chat_history had no index on session_id — only the primary key — so every lookup of
+a conversation's messages was a sequential scan. The columns match the ordering
+every reader now uses, (created_at, id), so the inbox previews and
+get_session_history are both answered from the index alone.
+"""
+from alembic import op
+import sqlalchemy as sa
+
+# revision identifiers, used by Alembic.
+revision = 'add_chat_session_idx_001'
+down_revision = 'hc_img_paths_001'
+branch_labels = None
+depends_on = None
+
+INDEX_NAME = 'ix_chat_history_session_created'
+
+
+def upgrade() -> None:
+    op.create_index(
+        INDEX_NAME,
+        'chat_history',
+        ['session_id', sa.text('created_at DESC'), sa.text('id DESC')],
+        unique=False,
+    )
+
+
+def downgrade() -> None:
+    op.drop_index(INDEX_NAME, table_name='chat_history')