Require explicit http.Client in podcast service (DIP)

Remove the silent nil-fallback in NewPodcastService /
NewPodcastServiceWithLogger that fabricated a default http.Client when
callers passed nil. The service now panics on nil, forcing callers to
inject their own client and own the HTTP timeout / transport policy
explicitly. DefaultHTTPClientTimeout remains exported so production
wiring (cmd/player/main.go) keeps a sensible default at the composition
root, not buried in the service.

Updated TestPodcastService_NilHTTPClient_Defaults to
TestPodcastService_NilHTTPClient_Panics to document the new contract.
All production call sites already pass a non-nil client.

Refs task 8a.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

2 files changed, 21 insertions, 10 deletions

diff --git a/player-server/internal/service/podcast.go b/player-server/internal/service/podcast.go
index 5adc0dc..bb3f0fd 100644
--- a/player-server/internal/service/podcast.go
+++ b/player-server/internal/service/podcast.go
@@ -79,16 +79,24 @@ type podcastService struct {
 	*podcastFeedChecker
 }
 
-// DefaultHTTPClientTimeout is the fallback timeout used when no http.Client is injected.
+// DefaultHTTPClientTimeout is the recommended timeout for production HTTP clients
+// passed into the podcast service. The service no longer constructs a fallback
+// client — callers must inject an explicit *http.Client (dependency inversion);
+// this constant is exported so production wiring can use a sensible default.
 const DefaultHTTPClientTimeout = 30 * time.Second
 
 // NewPodcastService creates a PodcastService with the given dependencies.
 // checkInterval should be the number of minutes between background feed checks.
+// httpClient is required and must not be nil; the service does not construct
+// a default client so callers explicitly own the HTTP timeout / transport policy.
 func NewPodcastService(store PodcastServiceStore, clk clock.Clock, mediaRoot string, helper *accessHelper, prober probe.Prober, thumbGen thumb.Generator, httpClient *http.Client, checkInterval int) *podcastService {
 	return NewPodcastServiceWithLogger(store, clk, mediaRoot, helper, prober, thumbGen, httpClient, checkInterval, slog.Default())
 }
 
 // NewPodcastServiceWithLogger creates a PodcastService with an injected logger.
+// httpClient is required and must not be nil — passing nil will panic on first
+// use. This is intentional: the service depends on an injected client per DIP
+// and refuses to silently fabricate one.
 func NewPodcastServiceWithLogger(store PodcastServiceStore, clk clock.Clock, mediaRoot string, helper *accessHelper, prober probe.Prober, thumbGen thumb.Generator, httpClient *http.Client, checkInterval int, logger *slog.Logger) *podcastService {
 	if checkInterval <= 0 {
 		checkInterval = 60
@@ -97,7 +105,7 @@ func NewPodcastServiceWithLogger(store PodcastServiceStore, clk clock.Clock, med
 		logger = slog.Default()
 	}
 	if httpClient == nil {
-		httpClient = &http.Client{Timeout: DefaultHTTPClientTimeout}
+		panic("service.NewPodcastService: httpClient must not be nil")
 	}
 	s := &podcastService{
 		store:         store,
diff --git a/player-server/internal/service/podcast_test.go b/player-server/internal/service/podcast_test.go
index b3c622e..2c86e99 100644
--- a/player-server/internal/service/podcast_test.go
+++ b/player-server/internal/service/podcast_test.go
@@ -40,17 +40,20 @@ func TestPodcastService_CustomHTTPClient(t *testing.T) {
 	}
 }
 
-func TestPodcastService_NilHTTPClient_Defaults(t *testing.T) {
+// TestPodcastService_NilHTTPClient_Panics verifies the constructor refuses to
+// fabricate a default http.Client. Callers must inject one explicitly (DIP);
+// passing nil is a programmer error and must panic loudly, not silently fall
+// back to a hidden default.
+func TestPodcastService_NilHTTPClient_Panics(t *testing.T) {
 	store := repository.NewMockStore()
 	clk := &clock.MockClock{T: time.Date(2024, 1, 1, 0, 0, 0, 0, time.UTC)}
 	logger := slog.New(slog.NewTextHandler(io.Discard, nil))
-	svc := NewPodcastServiceWithLogger(store, clk, t.TempDir(), nil, nil, nil, nil, 60, logger)
-	if svc.httpClient == nil {
-		t.Fatal("expected non-nil httpClient when nil passed to constructor")
-	}
-	if svc.httpClient.Timeout != DefaultHTTPClientTimeout {
-		t.Fatalf("expected default timeout %v, got %v", DefaultHTTPClientTimeout, svc.httpClient.Timeout)
-	}
+	defer func() {
+		if r := recover(); r == nil {
+			t.Fatal("expected panic when nil httpClient passed to constructor, got none")
+		}
+	}()
+	_ = NewPodcastServiceWithLogger(store, clk, t.TempDir(), nil, nil, nil, nil, 60, logger)
 }
 
 func TestPodcastService_SubscribeFeed_Ok(t *testing.T) {