Backend: add check_config coverage for stale config key references

check_config() only runs at app boot (app.py), so the rest of the test
suite never exercises it. A reference to a key removed from CONFIG_OPTIONS
(e.g. a toggle migrated to a feature flag) raises KeyError at startup but
is invisible to CI. This test runs check_config() against a fully-populated
config in both dev and prod modes to catch that class of bug.

This commit intentionally contains only the test (not the fix), so CI shows
it failing against the current ENABLE_DONATIONS KeyError on staging.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: aapeliv

app/backend/src/tests/test_config.py

@@ -0,0 +1,44 @@
+from typing import Any
+
+import pytest
+
+from couchers.config import CONFIG_OPTIONS, check_config
+
+
+def _complete_config(dev: bool) -> dict[str, Any]:
+    """Build a config dict with every CONFIG_OPTIONS key populated with a valid, truthy value.
+
+    This mirrors what make_config() produces when every env var is set, so check_config() must be
+    able to run against it without touching any key outside CONFIG_OPTIONS.
+    """
+    cfg: dict[str, Any] = {}
+    for name, type_, *_ in CONFIG_OPTIONS:
+        if type_ is bool:
+            cfg[name] = True
+        elif type_ is int:
+            cfg[name] = 1
+        elif type_ is bytes:
+            cfg[name] = b"x"
+        elif isinstance(type_, list):
+            cfg[name] = type_[0]
+        else:
+            cfg[name] = "x"
+
+    cfg["DEV"] = dev
+    if not dev:
+        # production invariants that aren't satisfiable by a generic truthy value
+        cfg["BASE_URL"] = "https://example.com"
+        cfg["ENABLE_EMAIL"] = True
+        cfg["IN_TEST"] = False
+    return cfg
+
+
+@pytest.mark.parametrize("dev", [True, False])
+def test_check_config_only_references_known_keys(dev):
+    """check_config() must only access config keys that are declared in CONFIG_OPTIONS.
+
+    A reference to a key that was removed from CONFIG_OPTIONS (e.g. a toggle migrated to a feature
+    flag) would raise KeyError at app boot but is invisible to the rest of the test suite, since
+    check_config() only runs in app.py's startup path. Exercising it here catches that.
+    """
+    check_config(_complete_config(dev=dev))