feat: Initial CBPOA commit — 武汉儿童呼吸疾病风险评估系统

Context: Build a spatial risk assessment system correlating air quality
data with children's respiratory disease incidence across Wuhan.

Approach: FastAPI backend serving PostGIS spatial queries, React
frontend with Deck.gl maps, and a PyTorch SpatialTemporalGCN pipeline
for multi-day (1d/3d/7d) risk prediction.

Changes:
- backend/ — FastAPI API with auth (JWT), alerts, risk analysis,
  geocoded case data, grid statistics, and report endpoints
- frontend/ — React dashboard with interactive risk maps, alert
  monitoring, district comparison charts, and timeline player
- models/ — SpatialTemporalGCN model with trained weights and ONNX
  export for inference
- scripts/ — ETL pipeline for weather + medical data, grid generation,
  feature engineering, training, and daily inference
- deploy/ — Docker Compose configs for backend, frontend, and MLflow
- docs/ — API docs, deployment guide, user guide, and code review

Impact: Enables spatial risk visualization, alert monitoring, and
ML-driven health risk forecasting for environmental health teams.

backend/database.py

@@ -0,0 +1,94 @@
+"""
+Database connection and session management for PostGIS
+"""
+import logging
+import asyncpg
+from typing import Optional
+from contextlib import asynccontextmanager
+from pydantic_settings import BaseSettings
+
+logger = logging.getLogger("cbpoa.database")
+
+
+class Settings(BaseSettings):
+    """Database settings from environment variables"""
+    POSTGRES_HOST: str = "localhost"
+    POSTGRES_PORT: int = 5432
+    POSTGRES_USER: str = ""
+    POSTGRES_PASSWORD: str = ""
+    POSTGRES_DB: str = ""
+    
+    class Config:
+        env_file = ".env"
+
+
+settings = Settings()
+
+if not settings.POSTGRES_USER or not settings.POSTGRES_PASSWORD or not settings.POSTGRES_DB:
+    raise RuntimeError(
+        "Missing required database environment variables: POSTGRES_USER, POSTGRES_PASSWORD, POSTGRES_DB. "
+        "Create a .env file or set them in your environment."
+    )
+
+
+class Database:
+    """Async database connection pool manager"""
+    
+    def __init__(self):
+        self.pool: Optional[asyncpg.Pool] = None
+    
+    async def connect(self):
+        """Initialize database connection pool"""
+        if self.pool is None:
+            dsn = f"postgresql://{settings.POSTGRES_USER}:{settings.POSTGRES_PASSWORD}@{settings.POSTGRES_HOST}:{settings.POSTGRES_PORT}/{settings.POSTGRES_DB}"
+            self.pool = await asyncpg.create_pool(
+                dsn=dsn,
+                min_size=5,
+                max_size=20,
+                command_timeout=60
+            )
+            logger.info("Database connection pool created successfully")
+    
+    async def disconnect(self):
+        """Close database connection pool"""
+        if self.pool:
+            await self.pool.close()
+            self.pool = None
+            logger.info("Database connection pool closed")
+    
+    @asynccontextmanager
+    async def get_connection(self):
+        """Get a connection from the pool"""
+        if self.pool is None:
+            await self.connect()
+        
+        async with self.pool.acquire() as connection:
+            yield connection
+    
+    @asynccontextmanager
+    async def get_transaction(self):
+        """Get a transaction context"""
+        if self.pool is None:
+            await self.connect()
+        
+        async with self.pool.acquire() as connection:
+            async with connection.transaction():
+                yield connection
+
+
+# Global database instance
+db = Database()
+
+
+async def init_db():
+    """Initialize database on startup - graceful degradation if unavailable"""
+    try:
+        await db.connect()
+    except Exception as e:
+        logger.warning("Database not available (%s). Running in demo mode.", e)
+        logger.warning("Set POSTGRES_HOST/POSTGRES_USER/POSTGRES_PASSWORD environment variables for database access.")
+
+
+async def close_db():
+    """Close database on shutdown"""
+    await db.disconnect()