Add Phase 8: lesson plan templates with deep-copy instantiation

- New tables: lesson_plan_template, lesson_plan_template_section, lesson_plan_template_item
- skill_level enum: beginner, intermediate, advanced, all_levels
- Templates are reusable curriculum definitions independent of any member/enrollment
- POST /lesson-plan-templates/:id/create-plan deep-copies the template into a member plan
- Instantiation uses template name as default plan title, accepts custom title override
- Instantiation deactivates any existing active plan on the enrollment (one-active rule)
- Plan items are independent copies — renaming the template does not affect existing plans
- 11 new integration tests

packages/backend/src/db/migrations/meta/_journal.json

@@ -253,6 +253,13 @@
       "when": 1774950000000,
       "tag": "0035_grade_history",
       "breakpoints": true
+    },
+    {
+      "idx": 36,
+      "version": "7",
+      "when": 1774960000000,
+      "tag": "0036_lesson_plan_templates",
+      "breakpoints": true
     }
   ]
 }