feat(api): GET /api/products/export — streamed canonical CSV, 409 empty_catalog (§6.4, PUC-9)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

backend/app/main.py

@@ -17,7 +17,7 @@ from typing import Any
 
 import psycopg
 from fastapi import Depends, FastAPI, File, Query, Response, UploadFile
-from fastapi.responses import JSONResponse, PlainTextResponse
+from fastapi.responses import JSONResponse, PlainTextResponse, StreamingResponse
 from fastapi.staticfiles import StaticFiles
 from pydantic import BaseModel
 
@@ -374,6 +374,27 @@ def create_app(database_url: str | None = None, static_dir: str | Path | None =
         _account, sf = gate
         return products.summary(conn, sf.id)
 
+    @app.get("/api/products/export")
+    def export_products(
+        status: str = Query(default="all", pattern="^(all|active|draft|archived)$"),
+        conn: psycopg.Connection = Depends(get_conn),
+        sess: dict | None = Depends(get_session),
+    ):
+        """Stream the catalog as canonical CSV, optionally status-filtered (§6.4; PUC-9)."""
+        gate = _merchant_gate(conn, sess)
+        if isinstance(gate, JSONResponse):
+            return gate
+        _account, sf = gate
+        try:
+            stream = products.export_catalog(conn, sf.id, status)
+        except products.EmptyCatalog:
+            return _error(409, "empty_catalog", "There are no products to export.")
+        return StreamingResponse(
+            stream,
+            media_type="text/csv",
+            headers={"content-disposition": 'attachment; filename="ecomm-products-export.csv"'},
+        )
+
     @app.get("/api/products/sample.csv")
     def products_sample_csv():
         """The DOC-3 worked-example CSV. Documentation, so no auth gate (§6.4)."""