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>

1 files changed, 5 insertions, 3 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 {