Remove option config.services.tt-rss.checkForUpdates (forced to false)

Force this option to false. Leaving this as true (currently the default)
is dangerous. If the TT-RSS installation upgrades itself to a newer
version requiring a schema update, the installation will break the next
time the TT-RSS systemd service is restarted.

Ideally, the installation itself should be immutable (see
https://github.com/NixOS/nixpkgs/issues/55300).

nixos/modules/services/web-apps/tt-rss.nix

@@ -46,7 +46,17 @@ let
       define('SINGLE_USER_MODE', ${boolToString cfg.singleUserMode});
 
       define('SIMPLE_UPDATE_MODE', ${boolToString cfg.simpleUpdateMode});
-      define('CHECK_FOR_UPDATES', ${boolToString cfg.checkForUpdates});
+
+      // Never check for updates - the running version of the code should be
+      // controlled entirely by the version of TT-RSS active in the current Nix
+      // profile. If TT-RSS updates itself to a version requiring a database
+      // schema upgrade, and then the SystemD tt-rss.service is restarted, the
+      // old code copied from the Nix store will overwrite the updated version,
+      // causing the code to detect the need for a schema "upgrade" (since the
+      // schema version in the database is different than in the code), but the
+      // update schema operation in TT-RSS will do nothing because the schema
+      // version in the database is newer than that in the code.
+      define('CHECK_FOR_UPDATES', false);
 
       define('FORCE_ARTICLE_PURGE', ${toString cfg.forceArticlePurge});
       define('SESSION_COOKIE_LIFETIME', ${toString cfg.sessionCookieLifetime});
@@ -399,14 +409,6 @@ let
         '';
       };
 
-      checkForUpdates = mkOption {
-        type = types.bool;
-        default = true;
-        description = ''
-          Check for updates automatically if running Git version
-        '';
-      };
-
       enableGZipOutput = mkOption {
         type = types.bool;
         default = true;
@@ -474,6 +476,14 @@ let
     };
   };
 
+  imports = [
+    (mkRemovedOptionModule ["services" "tt-rss" "checkForUpdates"] ''
+      This option was removed because setting this to true will cause TT-RSS
+      to be unable to start if an automatic update of the code in
+      services.tt-rss.root leads to a database schema upgrade that is not
+      supported by the code active in the Nix store.
+    '')
+  ];
 
   ###### implementation