fix(speak): honour the voice profile's language instead of forcing English

Both speak surfaces built their GenerationRequest with a hardcoded "en"
fallback and never consulted the resolved profile, so a profile created
with language="fr" was still synthesised as English unless the caller
passed language= explicitly.

This hurts the MCP path most: an agent calling voicebox.speak has no way
to know the bound profile's language, so it cannot pass the argument
either. Every agent-triggered generation on a non-English profile came
out with an English accent.

The fallback chain is now explicit argument -> resolved profile's
language -> "en", which matches how engine and personality already
consult the resolved binding. The "en" backstop is kept so profiles with
no language set behave exactly as before.

Adds backend/tests/test_speak_language.py covering both surfaces: the
fallback, explicit-argument precedence, and the unchanged "en" default.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>

backend/routes/speak.py

@@ -75,7 +75,7 @@ async def speak(
         models.GenerationRequest(
             profile_id=profile.id,
             text=data.text,
-            language=data.language or "en",
+            language=data.language or profile.language or "en",
             engine=engine,
             personality=bool(personality_flag),
         ),