Enhance credential management and testing functionality

- Introduce new fields for voice, speed, and language in the AssistantConfig and ProviderCredential models to support TTS and ASR configurations.
- Update the database schema and seeding script to accommodate the new fields, ensuring backward compatibility.
- Implement credential testing endpoints and logic to validate OpenAI-compatible credentials, enhancing user experience and reliability.
- Modify frontend components to include new fields in the credential forms and improve connection testing feedback.
- Refactor related services and API interactions to support the new credential testing feature.

backend/db/session.py

@@ -9,6 +9,7 @@ from collections.abc import AsyncGenerator
 
 import config
 from db.models import Base
+from sqlalchemy import text
 from sqlalchemy.ext.asyncio import (
     AsyncSession,
     async_sessionmaker,
@@ -27,3 +28,22 @@ async def get_session() -> AsyncGenerator[AsyncSession, None]:
 async def init_db() -> None:
     async with engine.begin() as conn:
         await conn.run_sync(Base.metadata.create_all)
+        # MVP 兼容迁移:create_all 不会给已存在的表补列。
+        await conn.execute(
+            text(
+                "ALTER TABLE provider_credentials "
+                "ADD COLUMN IF NOT EXISTS voice VARCHAR(128) NOT NULL DEFAULT ''"
+            )
+        )
+        await conn.execute(
+            text(
+                "ALTER TABLE provider_credentials "
+                "ADD COLUMN IF NOT EXISTS speed DOUBLE PRECISION NOT NULL DEFAULT 1.0"
+            )
+        )
+        await conn.execute(
+            text(
+                "ALTER TABLE provider_credentials "
+                "ADD COLUMN IF NOT EXISTS language VARCHAR(32) NOT NULL DEFAULT ''"
+            )
+        )