fix: the release preview sends the app's User-Agent, which Commons requires

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

tool/preview_release.py

@@ -33,6 +33,7 @@ from typing import Final
 import httpx
 
 from signal_bot.announce import render
+from signal_bot.app import HTTP_TIMEOUT_SECONDS, USER_AGENT
 from signal_bot.changelog import load, since
 from signal_bot.config_env import load_config
 from signal_bot.http_client import ImageFetcher
@@ -109,9 +110,16 @@ async def send_preview(group: str, ordinal: int, *, print_only: bool) -> None:
     if print_only:
         sys.stdout.write("== not sent (--print-only)\n")
         return
+    sys.stdout.flush()
 
     config = load_config()
-    async with httpx.AsyncClient(timeout=60.0) as client:
+    # The app's client, headers and all: Wikimedia answers 403 to httpx's
+    # default User-Agent, so a bare client here would fail exactly where the
+    # bot succeeds -- which is the opposite of what a preview is for.
+    async with httpx.AsyncClient(
+        timeout=HTTP_TIMEOUT_SECONDS,
+        headers={"User-Agent": USER_AGENT},
+    ) as client:
         group_id = await group_id_for(client, config.groups_url, group)
         portraits = PortraitCache(ImageFetcher(client), config.commons_api_url)
         image = await portraits.get_base64(pope)