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/app/database.py

@@ -0,0 +1,65 @@
+"""
+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.
+"""
+
+import os
+
+from sqlalchemy import create_engine
+from sqlalchemy.ext.declarative import declarative_base
+from sqlalchemy.orm import sessionmaker
+from app.core.config import settings
+from sqlalchemy.pool import QueuePool
+
+# Create SQLAlchemy engine with Unicode support
+# t3.micro RDS has ~45-65 max connections, reserve some for admin and other services
+engine = create_engine(
+    settings.DATABASE_URL,
+    pool_pre_ping=True,
+    poolclass=QueuePool,
+    pool_size=5,  # Conservative pool size for t3.micro
+    max_overflow=10,  # Allow up to 15 total connections from this pool
+    pool_timeout=30,  # Timeout waiting for connection
+    pool_recycle=1800,  # Recycle connections after 30 minutes
+    # Unicode, and UTC: naive timestamp columns default to now(), which is the
+    # session's local time. The code reads them as UTC, so pin the session.
+    connect_args={"options": "-c client_encoding=utf8 -c timezone=UTC"} if "postgresql" in settings.DATABASE_URL else {},
+    json_serializer=lambda obj: __import__('json').dumps(obj, ensure_ascii=False),
+    json_deserializer=lambda s: __import__('json').loads(s)
+)
+
+
+
+def _fresh_pool_after_fork() -> None:
+    """gunicorn --preload (scripts/start.sh) imports the app, which opens pooled
+    connections, before it forks the workers. A worker must never reuse its
+    parent's sockets: two workers talking over one connection corrupt each
+    other's transactions. So each child starts with an empty pool, leaving the
+    parent's connections open for the parent."""
+    engine.dispose(close=False)
+
+
+os.register_at_fork(after_in_child=_fresh_pool_after_fork)
+
+SessionLocal = sessionmaker(autocommit=False, autoflush=False, bind=engine)
+
+Base = declarative_base()
+
+
+def get_db():
+    db = SessionLocal()
+    try:
+        yield db
+    finally:
+        db.close()