Remove nil streamer fallback in api.serveFileResult (DIP)

The serveFileResult handler used to fabricate a default MediaStreamer at
request time when s.streamer was nil. That silently masked wiring
mistakes and violated the Dependency Inversion Principle — the handler
was inventing its own dependency instead of demanding one from the
caller. Removed the fallback so the handler now uses s.streamer directly.

Construction-time validation of deps.MediaStreamer was added to
api.NewServerWithLogger in commit 4215db6, which makes the runtime nil
case unreachable. Mirrors the explicit-deps pattern set in commits
622827c (http.Client in podcast service) and 92edb83 (TokenManager in
auth service).

Test helpers updated to inject a service.NewMediaStreamer(nil) when one
isn't otherwise supplied so the existing suite still constructs Servers
through the validating constructor.

Refs agent task 6a.

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

5 files changed, 22 insertions, 9 deletions

diff --git a/player-server/internal/api/handlers.go b/player-server/internal/api/handlers.go
index a9c832f..f04ec19 100644
--- a/player-server/internal/api/handlers.go
+++ b/player-server/internal/api/handlers.go
@@ -162,10 +162,12 @@ func (s *Server) serveDetach(w http.ResponseWriter, r *http.Request) {
 // ------------------------------------------------------------------
 
 func (s *Server) serveFileResult(w http.ResponseWriter, r *http.Request, res *service.FileResult, attachment bool) {
+	// s.streamer is required at construction time (see NewServerWithLogger),
+	// so it is guaranteed non-nil here. We previously fell back to a default
+	// streamer when nil, which silently hid wiring mistakes and violated the
+	// Dependency Inversion Principle by letting the handler decide its own
+	// dependency.
 	streamer := s.streamer
-	if streamer == nil {
-		streamer = service.NewMediaStreamer(nil)
-	}
 
 	stream, err := streamer.Open(r.Context(), res, attachment)
 	if err != nil {
diff --git a/player-server/internal/api/handlers_playback_test.go b/player-server/internal/api/handlers_playback_test.go
index 7febab0..869b0d1 100644
--- a/player-server/internal/api/handlers_playback_test.go
+++ b/player-server/internal/api/handlers_playback_test.go
@@ -38,7 +38,8 @@ func newPlaybackTestServer(t *testing.T, store repository.Store, sm auth.Session
 			Auth:          authSvc,
 			PlaybackHints: hintSvc,
 		},
-		StaticFS: fs,
+		StaticFS:      fs,
+		MediaStreamer: service.NewMediaStreamer(nil),
 	})
 	if err != nil {
 		t.Fatalf("NewServer: %v", err)
diff --git a/player-server/internal/api/handlers_podcast_test.go b/player-server/internal/api/handlers_podcast_test.go
index 4138d97..7dcdfe6 100644
--- a/player-server/internal/api/handlers_podcast_test.go
+++ b/player-server/internal/api/handlers_podcast_test.go
@@ -68,7 +68,8 @@ func newPodcastTestServer(t *testing.T, store repository.Store, hasher auth.Hash
 			Auth:     authSvc,
 			Podcast:  podcastSvc,
 		},
-		StaticFS: fs,
+		StaticFS:      fs,
+		MediaStreamer: service.NewMediaStreamer(nil),
 	})
 	if err != nil {
 		t.Fatalf("NewServer: %v", err)
diff --git a/player-server/internal/api/handlers_test.go b/player-server/internal/api/handlers_test.go
index 6279895..5fbd4c9 100644
--- a/player-server/internal/api/handlers_test.go
+++ b/player-server/internal/api/handlers_test.go
@@ -65,9 +65,17 @@ func newTestServer(t *testing.T, store repository.Store, hasher auth.Hasher, sm
 	if len(streamer) > 0 {
 		mediaStreamer = streamer[0]
 	}
-	// NewServer now returns an error when required deps (e.g. Config) are
-	// missing. Tests always pass a non-nil Config, so a failure here indicates
-	// a programming mistake in the test setup itself.
+	if mediaStreamer == nil {
+		// NewServer now requires a non-nil MediaStreamer at construction
+		// (see api.NewServerWithLogger). Tests that don't exercise streaming
+		// still need one, so we fall back to the default in-process streamer
+		// (nil remuxer means remux requests will error, which is fine for
+		// non-streaming tests).
+		mediaStreamer = service.NewMediaStreamer(nil)
+	}
+	// NewServer now returns an error when required deps (e.g. Config,
+	// MediaStreamer) are missing. Tests always pass non-nil values, so a
+	// failure here indicates a programming mistake in the test setup itself.
 	srv, err := NewServer(ServerDeps{
 		Store:          store,
 		Hasher:         hasher,
diff --git a/player-server/internal/api/integration_test.go b/player-server/internal/api/integration_test.go
index 2f51bd2..9344552 100644
--- a/player-server/internal/api/integration_test.go
+++ b/player-server/internal/api/integration_test.go
@@ -82,7 +82,8 @@ func newIntegrationServer(t *testing.T) *integrationEnv {
 			Auth:     authSvc,
 			Podcast:  &integrationPodcastService{},
 		},
-		StaticFS: http.FS(staticFS),
+		StaticFS:      http.FS(staticFS),
+		MediaStreamer: service.NewMediaStreamer(nil),
 	})
 	if err != nil {
 		t.Fatalf("NewServer: %v", err)