Add PostgreSQL connection retry configuration options

- Add retry environment variables
- Fix asyncpg import in retry tests

(cherry picked from commit bd535e3e7a)

env.example

@@ -344,6 +344,16 @@ POSTGRES_HNSW_M=16
 POSTGRES_HNSW_EF=200
 POSTGRES_IVFFLAT_LISTS=100
 
+### PostgreSQL Connection Retry Configuration (Network Robustness)
+### Number of retry attempts (1-10, default: 3)
+### Initial retry backoff in seconds (0.1-5.0, default: 0.5)
+### Maximum retry backoff in seconds (backoff-60.0, default: 5.0)
+### Connection pool close timeout in seconds (1.0-30.0, default: 5.0)
+# POSTGRES_CONNECTION_RETRIES=3
+# POSTGRES_CONNECTION_RETRY_BACKOFF=0.5
+# POSTGRES_CONNECTION_RETRY_BACKOFF_MAX=5.0
+# POSTGRES_POOL_CLOSE_TIMEOUT=5.0
+
 ### PostgreSQL SSL Configuration (Optional)
 # POSTGRES_SSL_MODE=require
 # POSTGRES_SSL_CERT=/path/to/client-cert.pem