BE-PR-001: Rename Propose -> Proposal across backend

- Backend: new canonical Proposal model + /proposals router
- Backward-compat shims for old Propose imports and /proposes API
- Tests updated to use /proposals, legacy compat tests added
- Impact checklist: docs/BE-PR-001-rename-impact.md
- TASKLIST.md: BE-PR-001 marked complete

All 21 proposal tests pass.

plans/automation/cron/hf-dev-kimi-scheduler-state.json

@@ -0,0 +1,8 @@
+{
+  "createdAt": "2026-03-29T11:50:00Z",
+  "lastRun": null,
+  "totalRuns": 0,
+  "completedTasks": [],
+  "currentTask": null,
+  "status": "active"
+}