feat: add data export/import feature

[victorprouff]

Summary

• Add GET /api/data/export: dumps all user data (family members, tasks, recipes, meal plans, budget entries & limits, shopping items, appointments, schedule entries) as a versioned JSON file
• Add POST /api/data/import: restores data from an export file inside a transaction; inserts are idempotent (ON CONFLICT DO NOTHING) so existing data is never overwritten
• Import order respects foreign key constraints (family_members and recipes first)
• Add a Settings page (/settings) with Export and Import UI cards
• Register /settings route and add entry in sidebar navigation

Use cases

• Backup & restore: export before a major update, reimport if something goes wrong
• Migration: export from one instance, import into another
• Schema changes: when the data model evolves, the JSON export can be transformed manually before reimport

Export format

{
  "version": "1.0",
  "exportedAt": "2026-03-08T12:00:00.000Z",
  "family_members": [...],
  "tasks": [...],
  "recipes": [...],
  "meal_plans": [...],
  "budget_entries": [...],
  "budget_limits": [...],
  "shopping_items": [...],
  "appointments": [...],
  "schedule_entries": [...]
}

Test plan

• Export triggers a JSON file download with all entities populated
• Import from a previously exported file restores all data correctly
• Re-importing the same file produces no duplicates (idempotent)
• Import failure rolls back entirely (no partial data)
• Settings page accessible from sidebar on desktop and mobile

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