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/admin/ai-settings/route.ts

@@ -1,3 +1,4 @@
+import { envOr } from "@/lib/env";
 import { NextRequest } from "next/server";
 import { apiHandler, requireAdmin, HttpError } from "@/lib/api-handler";
 import { prisma } from "@/lib/db";
@@ -95,7 +96,7 @@ export const PUT = apiHandler(async (request: NextRequest) => {
       "api.groq.com",
       "openrouter.ai",
     ]);
-    for (const h of (process.env.ADMIN_AI_BASE_URL_ALLOWLIST ?? "")
+    for (const h of envOr("ADMIN_AI_BASE_URL_ALLOWLIST", "")
       .split(",")
       .map((h) => h.trim())
       .filter(Boolean)) {