Migrate LISTMONK_ENABLED to a listmonk_enabled feature flag

Replace the static LISTMONK_ENABLED env config with a global feature flag
so Listmonk can be toggled remotely without a deploy. Listmonk credentials
stay in env and are now required in production unconditionally.

Author: aapeliv

app/backend.dev.env

@@ -58,7 +58,6 @@ BUG_TOOL_GITHUB_REPO=user/repo
 BUG_TOOL_GITHUB_USERNAME=user
 BUG_TOOL_GITHUB_TOKEN=token
 
-LISTMONK_ENABLED=0
 LISTMONK_BASE_URL=https://localhost
 LISTMONK_API_USERNAME=...
 LISTMONK_API_KEY=...

app/backend/src/couchers/config.py

@@ -95,8 +95,7 @@
     ("PUSH_NOTIFICATIONS_VAPID_SUBJECT", str),
     # Whether to initiate new activeness probes
     ("ACTIVENESS_PROBES_ENABLED", bool),
-    # Listmonk (mailing list)
-    ("LISTMONK_ENABLED", bool),
+    # Listmonk (mailing list, gated at runtime by the `listmonk_enabled` feature flag)
     ("LISTMONK_BASE_URL", str),
     ("LISTMONK_API_USERNAME", str),
     ("LISTMONK_API_KEY", str),
@@ -150,6 +149,16 @@ def check_config(cfg: dict[str, Any]) -> None:
         if not cfg["STRIPE_API_KEY"] or not cfg["STRIPE_WEBHOOK_SECRET"] or not cfg["STRIPE_RECURRING_PRODUCT_ID"]:
             raise Exception("Stripe credentials must be configured in production")
 
+        # Listmonk is gated at runtime by the `listmonk_enabled` feature flag, which can be flipped on
+        # remotely at any time, so prod must always have the Listmonk credentials present.
+        if (
+            not cfg["LISTMONK_BASE_URL"]
+            or not cfg["LISTMONK_API_USERNAME"]
+            or not cfg["LISTMONK_API_KEY"]
+            or not cfg["LISTMONK_LIST_ID"]
+        ):
+            raise Exception("Listmonk credentials must be configured in production")
+
         # The following features are gated at runtime by feature flags (`strong_verification_enabled`,
         # `postal_verification_enabled`, `recaptcha_enabled`), which can be flipped on remotely at any
         # time, so prod must always have their credentials present.

app/backend/src/couchers/jobs/handlers.py

@@ -614,7 +614,7 @@ def send_host_request_reminders(payload: empty_pb2.Empty) -> None:
 
 
 def add_users_to_email_list(payload: empty_pb2.Empty) -> None:
-    if not config["LISTMONK_ENABLED"]:
+    if not experimentation.get_global_boolean_value("listmonk_enabled", default=False):
         logger.info("Not adding users to mailing list")
         return
 

app/backend/src/tests/conftest.py

@@ -209,7 +209,6 @@ def testconfig():
     config["BUG_TOOL_GITHUB_USERNAME"] = "user"
     config["BUG_TOOL_GITHUB_TOKEN"] = "token"
 
-    config["LISTMONK_ENABLED"] = False
     config["LISTMONK_BASE_URL"] = "https://localhost"
     config["LISTMONK_API_USERNAME"] = "..."
     config["LISTMONK_API_KEY"] = "..."

app/backend/src/tests/test_bg_jobs.py

@@ -1217,9 +1217,9 @@ def test_send_host_request_reminders(db, moderator):
             assert find in html, f"Expected to find string {find} in HTML email {subject} to {address}, didn't"
 
 
-def test_add_users_to_email_list(db):
+def test_add_users_to_email_list(db, feature_flags):
+    feature_flags.set("listmonk_enabled", True)
     new_config = config.copy()
-    new_config["LISTMONK_ENABLED"] = True
     new_config["LISTMONK_BASE_URL"] = "https://example.com"
     new_config["LISTMONK_API_USERNAME"] = "test_user"
     new_config["LISTMONK_API_KEY"] = "dummy_api_key"