Fix rate limiter header check bug

Fixed ArgumentError in rate limiter by replacing cond statement with
proper case pattern matching. The cond was incorrectly evaluating
assignment expressions as truthy values instead of checking if headers
were empty lists.

🤖 Generated with [Claude Code](https://claude.ai/code)

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

lib/aprsme_web/plugs/rate_limiter.ex

@@ -41,20 +41,22 @@ defmodule AprsmeWeb.Plugs.RateLimiter do
 
   defp get_key(conn, :ip) do
     # Check headers in order of preference
-    cond do
+    case {get_req_header(conn, "cf-connecting-ip"), get_req_header(conn, "x-forwarded-for"),
+          get_req_header(conn, "x-real-ip")} do
       # Cloudflare header takes precedence
-      cf_ip = get_req_header(conn, "cf-connecting-ip") != [] ->
-        hd(cf_ip)
+      {[cf | _], _, _} ->
+        cf
 
-      # Then standard forwarded headers
-      forwarded = get_req_header(conn, "x-forwarded-for") != [] ->
-        forwarded |> hd() |> String.split(",") |> List.first() |> String.trim()
+      # Then standard X-Forwarded-For header
+      {[], [forwarded | _], _} ->
+        forwarded |> String.split(",") |> List.first() |> String.trim()
 
-      real_ip = get_req_header(conn, "x-real-ip") != [] ->
-        hd(real_ip)
+      # Then X-Real-IP header
+      {[], [], [real | _]} ->
+        real
 
       # Fall back to remote_ip
-      true ->
+      {[], [], []} ->
         conn.remote_ip |> :inet.ntoa() |> to_string()
     end
   end