chore: add plan files for Phase 3 advanced features

plans/phase-3-advanced-features/phase-7-integration.md

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+# Phase 7: Integration & Polish
+
+**Status:** ⬜ Not Started
+**Parent plan:** [PLAN.md](./PLAN.md)
+**Domain:** fullstack
+
+## Tasks
+- [ ] Task 1: Fix all TypeScript/build errors
+- [ ] Task 2: Verify `npm run build` succeeds
+- [ ] Task 3: Verify `npm run check` passes
+- [ ] Task 4: Verify `npm run lint` passes
+- [ ] Task 5: Write tests for export/import services
+- [ ] Task 6: Write tests for discovery service
+- [ ] Task 7: Write tests for user preferences API
+- [ ] Task 8: Write tests for quick-add API
+- [ ] Task 9: Update seed script with sample data
+- [ ] Task 10: Verify all existing tests pass
+- [ ] Task 11: Run Prisma migrations
+
+## Handoff
+<!-- Final phase -->