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>

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