fix(database): 数据库连接 - 消费 libpq keepalive 参数并默认限制闲置事务 (#737)

pgx 不识别 DSN 中 libpq 的 keepalives* 参数，会作为服务端运行参数发送导致连接被拒；改为在建连前消费并应用到 TCP 拨号器。
未显式配置时为每条连接默认设置 idle_in_transaction_session_timeout=5min，避免卡住的事务长期持锁；DSN 显式配置时以 DSN 为准。
建连钩子先复制共享的 RuntimeParams，避免多条连接之间累积修改。

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

backend/internal/database/postgres_connection_linux_test.go

@@ -0,0 +1,81 @@
+package database
+
+import (
+	"net"
+	"os"
+	"strconv"
+	"sync"
+	"syscall"
+	"testing"
+)
+
+// clientSocketKeepalive 在当前进程中按端口找到客户端 socket，读取其 keepalive 选项。
+func clientSocketKeepalive(port int) ([4]int, bool) {
+	entries, err := os.ReadDir("/proc/self/fd")
+	if err != nil {
+		return [4]int{}, false
+	}
+	for _, entry := range entries {
+		fd, err := strconv.Atoi(entry.Name())
+		if err != nil {
+			continue
+		}
+		addr, err := syscall.Getsockname(fd)
+		inet, ok := addr.(*syscall.SockaddrInet4)
+		if err != nil || !ok || inet.Port != port {
+			continue
+		}
+		var values [4]int
+		for i, option := range [][2]int{
+			{syscall.SOL_SOCKET, syscall.SO_KEEPALIVE},
+			{syscall.IPPROTO_TCP, syscall.TCP_KEEPIDLE},
+			{syscall.IPPROTO_TCP, syscall.TCP_KEEPINTVL},
+			{syscall.IPPROTO_TCP, syscall.TCP_KEEPCNT},
+		} {
+			if values[i], err = syscall.GetsockoptInt(fd, option[0], option[1]); err != nil {
+				return [4]int{}, false
+			}
+		}
+		return values, true
+	}
+	return [4]int{}, false
+}
+
+func TestPostgresOpenAppliesLibpqKeepaliveToSocket(t *testing.T) {
+	for _, tc := range []struct {
+		name, params string
+		want         [4]int
+	}{
+		{"explicit", "keepalives_idle=90 keepalives_interval=20 keepalives_count=6", [4]int{1, 90, 20, 6}},
+		{"partial keeps Go defaults", "keepalives_idle=45", [4]int{1, 45, 15, 9}},
+		{"no params keeps Go defaults", "", [4]int{1, 15, 15, 9}},
+		{"disabled", "keepalives=0", [4]int{0, -1, -1, -1}},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			server := newFakePostgresServer(t)
+			var mu sync.Mutex
+			var observed [][4]int
+			server.onStartup = func(conn net.Conn) {
+				// 已收到启动消息，说明客户端拨号已完成且 socket 选项已设置。
+				values, ok := clientSocketKeepalive(conn.RemoteAddr().(*net.TCPAddr).Port)
+				if !ok {
+					values = [4]int{-2, -2, -2, -2}
+				}
+				mu.Lock()
+				observed = append(observed, values)
+				mu.Unlock()
+			}
+			openFakePostgres(t, server, tc.params, 0)
+			mu.Lock()
+			defer mu.Unlock()
+			for _, values := range observed {
+				if values[0] == -2 {
+					t.Fatal("client socket not found")
+				}
+				if values[0] != tc.want[0] || (tc.want[0] == 1 && values != tc.want) {
+					t.Fatalf("keepalive (enabled, idle, interval, count) = %v, want %v", values, tc.want)
+				}
+			}
+		})
+	}
+}