feat: US-007 - Comandă Discord /voice engine

- Comandă nouă /voice engine <pockettts|supertonic> setează tts.default_engine în config, persistă via Config().set+save()
- Helper _default_voice_for_engine() confirmă vocea implicită a engine-ului ales (alba pentru pockettts, voice.default_voice pentru supertonic)
- Gates: pytest PASS (22 eșecuri preexistente neschimbate), review manual PASS, wiring comandă verificat izolat (fără restart pe bot-ul live)

src/adapters/discord_voice.py

@@ -100,6 +100,13 @@ def _get_default_voice() -> str:
         return "M2"
 
 
+def _default_voice_for_engine(engine: str) -> str:
+    """Vocea implicită asociată engine-ului (nu vocea implicită globală per-guild)."""
+    if engine == "pockettts":
+        return "alba"
+    return _get_default_voice()
+
+
 def register(tree: app_commands.CommandTree, bot: discord.Client) -> app_commands.Group:
     """Build the `/voice` slash command group and return it (caller registers)."""
     voice_group = app_commands.Group(
@@ -285,6 +292,36 @@ def register(tree: app_commands.CommandTree, bot: discord.Client) -> app_command
             msg = f"Default voce setată {new_voice}. Va intra în vigoare la următorul /voice join."
         await interaction.followup.send(msg, ephemeral=True)
 
+    _ENGINE_CHOICES = [
+        app_commands.Choice(name="pocket-tts", value="pockettts"),
+        app_commands.Choice(name="Supertonic", value="supertonic"),
+    ]
+
+    @voice_group.command(name="engine", description="Schimbă engine-ul TTS implicit (pockettts/supertonic)")
+    @app_commands.describe(engine="Engine TTS")
+    @app_commands.choices(engine=_ENGINE_CHOICES)
+    async def engine_cmd(
+        interaction: discord.Interaction,
+        engine: app_commands.Choice[str],
+    ) -> None:
+        await interaction.response.defer(ephemeral=True)
+        new_engine = engine.value
+        try:
+            cfg = Config()
+            cfg.set("tts.default_engine", new_engine)
+            cfg.save()
+        except Exception as e:
+            log.warning("config save failed for tts.default_engine: %s", e)
+            await interaction.followup.send(
+                f"Eroare la salvarea engine-ului: {e}", ephemeral=True
+            )
+            return
+        default_voice = _default_voice_for_engine(new_engine)
+        await interaction.followup.send(
+            f"Engine TTS implicit setat pe **{new_engine}**. Voce implicită: {default_voice}.",
+            ephemeral=True,
+        )
+
     @voice_group.command(name="stop", description="Oprește audio-ul curent (golește coada TTS)")
     async def stop_audio(interaction: discord.Interaction) -> None:
         await interaction.response.defer(ephemeral=True)