fix(config): treat an empty environment variable as unset

docker-compose.yml forwards its whitelist as "${NAME:-}", so a variable
the operator never set reaches the process as the empty string. Reads
written as a nullish fallback kept that string: the weather client built
an empty base URL and every geocode failed with ERR_INVALID_URL, and the
IP geo lookup requested a relative path.

Reads with a fallback now go through src/lib/env.ts (envValue, envOr,
envFlag), which treats empty and whitespace as unset. IP_GEO_LOOKUP_DISABLED
accepts 1, true, yes and on instead of only "1". A structural test refuses
a ?? fallback written directly against process.env anywhere in src/, and
the compose whitelist guard counts the helper's call shape as a read.

src/app/api/settings/telegram/route.ts

@@ -1,3 +1,4 @@
+import { envValue } from "@/lib/env";
 import { prisma } from "@/lib/db";
 import {
   apiError,
@@ -50,7 +51,7 @@ export const GET = apiHandler(async () => {
 });
 
 async function registerTelegramWebhook(botToken: string) {
-  const appUrl = process.env.APP_URL ?? process.env.NEXT_PUBLIC_APP_URL;
+  const appUrl = envValue("APP_URL") ?? envValue("NEXT_PUBLIC_APP_URL");
   if (!appUrl) {
     return apiError(
       "Server configuration error: APP_URL (or NEXT_PUBLIC_APP_URL) is missing.",