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/mcp_server/tools.py

@@ -104,7 +104,7 @@ def register_tools(mcp: FastMCP) -> None:
                 profile_name=vp.name,
                 text=text,
                 engine=resolved_engine,
-                language=language,
+                language=language or vp.language,
                 personality=use_persona,
                 model_size=model_size,
                 db=db,