test: skip Redis-dependent tests in CI when Redis isn't reachable

The Forgejo CI test gate has no Redis service, so FourOhFourTracker
and RedisHealthCheck tests fail with Redix.ConnectionError. They've
always been broken in CI but the failures got noticed now.

Mirrors the existing :requires_gdal pattern: probe localhost:6379 in
test_helper.exs and add :requires_redis to extra_excludes when nothing
answers. Locally (where direnv runs Redis) the tag isn't excluded so
the tests still run.

test/test_helper.exs

@@ -47,13 +47,28 @@ ExUnit.start()
 # aren't installed (e.g. CI test gate runs in a lean image without
 # gdal-bin). Locally we install GDAL via `brew install gdal` per
 # CLAUDE.md, so the test runs there.
-extra_excludes =
+gdal_excludes =
   if System.find_executable("gdal_translate") && System.find_executable("gdaldem") do
     []
   else
     [:requires_gdal]
   end
 
+# Skip Redis-backed tests when Redis isn't reachable on localhost:6379
+# (e.g. the Forgejo CI test gate has no Redis service). Local dev runs
+# Redis via direnv/services so the tests run there.
+redis_excludes =
+  case :gen_tcp.connect(~c"127.0.0.1", 6379, [:binary, active: false], 250) do
+    {:ok, sock} ->
+      :gen_tcp.close(sock)
+      []
+
+    {:error, _} ->
+      [:requires_redis]
+  end
+
+extra_excludes = gdal_excludes ++ redis_excludes
+
 # Exclude tests by default
 # Run specific tests with: mix test --only <tag>
 ExUnit.configure(

test/towerops/redis_health_check_test.exs

@@ -5,6 +5,8 @@ defmodule Towerops.RedisHealthCheckTest do
 
   alias Towerops.RedisHealthCheck
 
+  @moduletag :requires_redis
+
   describe "wait_for_redis/2" do
     test "returns :ok when Redis is available" do
       redis_config = Application.get_env(:towerops, :redis, host: "localhost", port: 6379)

test/towerops/security/four_oh_four_tracker_test.exs

@@ -4,6 +4,7 @@ defmodule Towerops.Security.FourOhFourTrackerTest do
   alias Towerops.Security.FourOhFourTracker
 
   @moduletag :four_oh_four_tracker
+  @moduletag :requires_redis
 
   setup do
     # Ensure a real Redix connection exists for these tests.