Add POST /redrob/speech so a client can play speech without holding the key

[flxkm522]

Read-aloud, option (a) from the media-output plan: the engine speaks, and Redrob Cowork plays the result. Stacked on #71; base is feature/speech-generate-tool.

Why an engine route: read-aloud has to play audio in the app, but the Redrob key belongs to the engine. POST /redrob/speech speaks text through the gateway using the engine's own credential and returns the mp3. The key never leaves this process. Cowork's server will proxy this route to the app, the same way it already talks to the engine for /auth/redrob.

What changes

• POST /redrob/speech in the control group (authorized root API, next to /auth/:providerID).
  • Payload: { text, voice?, model? }.
  • Success: audio/mpeg bytes (HttpApiSchema.asUint8Array).
• One shared code path with speech_generate (src/tool/speech_generate.ts):
  • prepareSpeech validates the text and picks the model and voice.
  • speechKey finds the credential.
  • requestSpeech makes the single gateway call and checks the answer is audio.
  • Each fails with a typed SpeechError. The tool's behaviour is unchanged.
• Declared errors:

Case	Status	Sent to gateway?
empty text, or over 4,096 code points	400 InvalidRequestError	no
no Redrob credential	503 ServiceUnavailableError (the engine lacks it, not the caller)	no
gateway refusal, or a non-audio answer	502 UpstreamError with the gateway's message	yes
timeout	504 TimeoutError	yes

Proof

Command	Result
bun test test/server/httpapi-speech.test.ts	4 pass. Uses the production route tree on NodeHttpServer.layerTest with a gateway stand-in on its own Effect listener and fresh router. Checks: the mp3 bytes round-trip; the engine key goes only to the gateway as a bearer token; the exact gateway body; 400 and 503 with nothing sent; a 402 refusal and a JSON 200 both become 502 carrying the gateway's message
bun test test/tool/speech_generate.test.ts	4 pass, unchanged after the refactor
bun test test/server test/tool	740 pass / 1 fail. The failure is write.test.ts "OS denies write access", which also fails on develop here (runs as root). OpenAPI tests pass with the new route
bun typecheck (pre-push, 18 packages)	exit 0
prettier --check, oxlint on the changed files	clean

Not covered: a live call to the gateway (needs mckinley-and-rice/redrob-console#232 deployed).