Surface programming errors in user fetches

tests/test_main.py

@@ -65,6 +65,25 @@ async def test_fetch_source_usernames_skips_failed_accounts() -> None:
     assert stale is False
 
 
+@pytest.mark.asyncio
+async def test_fetch_source_usernames_does_not_swallow_programming_errors() -> None:
+    def broken_fetcher(username):
+        async def fetch():
+            raise TypeError("bad shape")
+
+        return fetch
+
+    with pytest.raises(TypeError, match="bad shape"):
+        await _fetch_source_usernames(
+            None,
+            "github",
+            ["octocat", "hubot"],
+            date(2026, 1, 1),
+            date(2026, 1, 1),
+            broken_fetcher,
+        )
+
+
 @pytest.mark.asyncio
 async def test_fetch_source_usernames_keeps_cache_after_username_list_changes() -> None:
     with tempfile.TemporaryDirectory(dir=Path.cwd()) as temp_dir: