fix: preserve speed_limit and auto_clear when saving forwards and user tunnels

- Add speed_limit and auto_clear fields to forward update mutation
- Update user tunnel save to preserve these fields
- Add contract test for forward save preserving speed_limit
- Add plan documents for the fixes

plans/006-forward-save-missing-speed-limit-auto-clear.md

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+# 006 Forward Save Missing Speed Limit Auto Clear
+
+## Checklist
+
+- [x] Locate forward create/update speed limit validation path that blocks save when speed rule is deleted.
+- [x] Change forward save behavior to auto-clear missing `speedId` instead of returning "限速规则不存在".
+- [x] Add contract test coverage for editing a forward after its referenced speed limit is deleted.
+- [x] Run focused contract tests for forward save behavior.

plans/007-user-tunnel-save-missing-speed-limit-auto-clear.md

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+# 007 User Tunnel Save Missing Speed Limit Auto Clear
+
+## Checklist
+
+- [x] Locate user tunnel speed limit validation paths for assign/update flows.
+- [x] Change user tunnel save behavior to auto-clear missing `speedId` instead of failing.
+- [x] Add contract test coverage for user tunnel save when referenced speed limit is deleted.
+- [x] Run focused contract tests for user tunnel save behavior.

plans/008-frontend-missing-speed-limit-consistency.md

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+# 008 Frontend Missing Speed Limit Consistency
+
+## Checklist
+
+- [x] Review forward and user tunnel submit flows for missing speed limit behavior.
+- [x] Make frontend normalize deleted `speedId` to `null` before submit in both pages.
+- [x] Add consistent non-blocking warning toast when deleted speed rule is auto-cleared.
+- [x] Verify touched frontend files pass lint checks.