feat: add PostgreSQL web installation wizard

oa-backend/src/main/java/com/kaidi/oa/web/HealthController.java

@@ -1,19 +1,41 @@
 package com.kaidi.oa.web;
 
 import com.kaidi.oa.common.ApiResp;
+import org.springframework.http.HttpStatus;
+import org.springframework.http.ResponseEntity;
 import org.springframework.web.bind.annotation.GetMapping;
 import org.springframework.web.bind.annotation.RequestMapping;
 import org.springframework.web.bind.annotation.RestController;
 
+import javax.sql.DataSource;
+import java.sql.Connection;
+import java.sql.ResultSet;
+import java.sql.Statement;
 import java.util.Map;
 
-/** Liveness probe. */
+/** Readiness probe used by the installer and online-update rollback gate. */
 @RestController
 @RequestMapping("/api/oa")
 public class HealthController {
 
+    private final DataSource dataSource;
+
+    public HealthController(DataSource dataSource) {
+        this.dataSource = dataSource;
+    }
+
     @GetMapping("/health")
-    public ApiResp<Map<String, String>> health() {
-        return ApiResp.ok(Map.of("status", "UP"));
+    public ResponseEntity<ApiResp<Map<String, String>>> health() {
+        try (Connection connection = dataSource.getConnection();
+                Statement statement = connection.createStatement();
+                ResultSet result = statement.executeQuery("SELECT 1")) {
+            if (connection.isValid(2) && result.next() && result.getInt(1) == 1) {
+                return ResponseEntity.ok(ApiResp.ok(Map.of("status", "UP", "database", "UP")));
+            }
+        } catch (Exception ignored) {
+            // Health responses deliberately avoid exposing connection details.
+        }
+        return ResponseEntity.status(HttpStatus.SERVICE_UNAVAILABLE)
+                .body(new ApiResp<>(50301, "database unavailable", Map.of("status", "DOWN", "database", "DOWN")));
     }
 }