fix(db): 修复 sourcemap 上传因唯一约束缺少 build_id 导致 HTTP 500

uk_map 约束为 (app_key, platform, app_version, bundle_name)，未包含
build_id。Gradle 插件每次构建生成不同 buildId，服务端查不到记录后尝试
INSERT，触发 Duplicate entry 约束冲突返回 500。

V8 migration：删除旧约束，重建为
(app_key, platform, app_version, build_id, bundle_name)。
MySQL UNIQUE 对 NULL 视为不相等，历史 build_id=NULL 记录不受影响；
应用层已通过 findByBuildIdIsNull 防止 NULL 记录重复插入。

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

xuqm-bugcollect-service/src/main/resources/db/migration/V8__sourcemap_uk_add_build_id.sql

@@ -0,0 +1,14 @@
+-- V8: Include build_id in log_sourcemaps unique constraint.
+--
+-- Problem: uk_map was (app_key, platform, app_version, bundle_name).
+-- When the Gradle plugin uploads mapping files with a per-build buildId,
+-- each new build of the same version generates a different buildId and tries
+-- to INSERT a new row, which violates the old constraint → HTTP 500.
+--
+-- Fix: rebuild uk_map to include build_id.
+-- MySQL treats NULLs as distinct in unique indexes, so legacy records
+-- (build_id IS NULL) coexist without conflict. Application logic already
+-- prevents duplicate NULL-buildId rows via findByAppKeyAndPlatformAndAppVersionAndBuildIdIsNullAndBundleName.
+
+ALTER TABLE log_sourcemaps DROP INDEX uk_map;
+ALTER TABLE log_sourcemaps ADD UNIQUE KEY uk_map (app_key, platform, app_version, build_id, bundle_name);