feat(export): JSON full-backup download (pantry + saved recipes)

Adds GET /export/json that bundles inventory and saved recipes into a
single timestamped JSON file for data portability. The export envelope
includes schema version and export timestamp so future import logic can
handle version differences.

Frontend: new primary-styled JSON download button in the Export card with
a short description of what is included.

Closes #62

app/api/endpoints/export.py

@@ -1,9 +1,11 @@
-"""Export endpoints — CSV/Excel of receipt and inventory data."""
+"""Export endpoints — CSV and JSON export of user data."""
 from __future__ import annotations
 
 import asyncio
 import csv
 import io
+import json
+from datetime import datetime, timezone
 
 from fastapi import APIRouter, Depends
 from fastapi.responses import StreamingResponse
@@ -45,3 +47,33 @@ async def export_inventory_csv(store: Store = Depends(get_store)):
         media_type="text/csv",
         headers={"Content-Disposition": "attachment; filename=inventory.csv"},
     )
+
+
+@router.get("/json")
+async def export_full_json(store: Store = Depends(get_store)):
+    """Export full pantry inventory + saved recipes as a single JSON file.
+
+    Intended for data portability — users can import this into another
+    Kiwi instance or keep it as an offline backup.
+    """
+    inventory, saved = await asyncio.gather(
+        asyncio.to_thread(store.list_inventory),
+        asyncio.to_thread(store.get_saved_recipes, 1000, 0),
+    )
+
+    export_doc = {
+        "kiwi_export": {
+            "version": "1.0",
+            "exported_at": datetime.now(timezone.utc).isoformat(),
+            "inventory": [dict(row) for row in inventory],
+            "saved_recipes": [dict(row) for row in saved],
+        }
+    }
+
+    body = json.dumps(export_doc, default=str, indent=2)
+    filename = f"kiwi-export-{datetime.now(timezone.utc).strftime('%Y%m%d')}.json"
+    return StreamingResponse(
+        iter([body]),
+        media_type="application/json",
+        headers={"Content-Disposition": f"attachment; filename={filename}"},
+    )

frontend/src/components/InventoryList.vue

@@ -392,10 +392,14 @@
     <!-- Export -->
     <div class="card export-card">
       <h2 class="section-title">Export</h2>
-      <div class="flex gap-sm" style="margin-top: var(--spacing-sm)">
+      <div class="flex gap-sm flex-wrap" style="margin-top: var(--spacing-sm)">
+        <button @click="exportJSON" class="btn btn-primary">Download JSON (full backup)</button>
         <button @click="exportCSV" class="btn btn-secondary">Download CSV</button>
         <button @click="exportExcel" class="btn btn-secondary">Download Excel</button>
       </div>
+      <p class="text-sm text-secondary" style="margin-top: var(--spacing-xs)">
+        JSON includes pantry + saved recipes. Import it into another Kiwi instance any time.
+      </p>
     </div>
 
     <!-- Edit Modal -->
@@ -847,6 +851,11 @@ function exportExcel() {
   window.open(`${apiUrl}/export/inventory/excel`, '_blank')
 }
 
+function exportJSON() {
+  const apiUrl = import.meta.env.VITE_API_URL || '/api/v1'
+  window.open(`${apiUrl}/export/json`, '_blank')
+}
+
 // Full date string for tooltip (accessible label)
 function formatDateFull(dateStr: string): string {
   const date = new Date(dateStr)