refactor(backend): migrate to modular repository pattern with separated concerns

- Extract database layer into model and repo packages
- Split repository into focused modules (control, federation, flow, groups, mutations)
- Remove monolithic db.go and sqlite/repository.go
- Update handlers to use new repository structure
- Migrate contract tests to new patterns
- Add migration plan documentation

.sisyphus/ralph-loop.local.md

@@ -0,0 +1,9 @@
+---
+active: true
+iteration: 1
+max_iterations: 100
+completion_promise: "DONE"
+started_at: "2026-02-15T16:38:58.212Z"
+session_id: "ses_39dd49703ffeveg711aA1D1YAk"
+---
+现状后端数据库兼容sqlite和postgresql，每一次新增功能需要维护两套数据库sql，需求是使用一个数据库驱动能同时兼容两个数据库，请仔细分析，列出计划，全量迁移，并且写好所有的测试，确保重构后所有的功能都能正常运行，由于工程量大，请写一个计划列表的markdown记录，每次完成一个就记录一下进度