fix: use underscore-separated MCP tool names so Claude Desktop accepts them

Claude Desktop validates tool names against ^[a-zA-Z0-9_-]{1,64}$ and
rejects the whole tool list when any name contains a dot, so the Voicebox
MCP server was unusable there. Rename voicebox.speak, voicebox.transcribe,
voicebox.list_captures and voicebox.list_profiles to voicebox_speak,
voicebox_transcribe, voicebox_list_captures and voicebox_list_profiles.

Fixes #790

backend/mcp_server/tools.py

@@ -36,7 +36,7 @@ def register_tools(mcp: FastMCP) -> None:
     """Attach all Voicebox tools to the given FastMCP instance."""
 
     @mcp.tool(
-        name="voicebox.speak",
+        name="voicebox_speak",
         description=(
             "Speak text in a Voicebox voice profile. Returns a generation id "
             "the caller can poll at /generate/{id}/status. Audio plays on the "
@@ -113,7 +113,7 @@ def register_tools(mcp: FastMCP) -> None:
             db.close()
 
     @mcp.tool(
-        name="voicebox.transcribe",
+        name="voicebox_transcribe",
         description=(
             "Transcribe an audio clip to text using Voicebox's local Whisper. "
             "Pass exactly one of `audio_base64` (bytes as base64) or "
@@ -171,7 +171,7 @@ def register_tools(mcp: FastMCP) -> None:
             tmp_path.unlink(missing_ok=True)
 
     @mcp.tool(
-        name="voicebox.list_captures",
+        name="voicebox_list_captures",
         description=(
             "List recent voice captures (dictations, recordings, uploads) "
             "with their transcripts. Most-recent first."
@@ -199,7 +199,7 @@ def register_tools(mcp: FastMCP) -> None:
             db.close()
 
     @mcp.tool(
-        name="voicebox.list_profiles",
+        name="voicebox_list_profiles",
         description=(
             "List available voice profiles (both cloned voices and presets). "
             "Use the returned `name` with voicebox.speak(profile=...)."