Add conversation history management and API endpoints

- Introduce new database models for conversation sessions, messages, and artifacts to support conversation history tracking.
- Implement API routes for listing conversations and retrieving detailed conversation data, enhancing user interaction with historical records.
- Add a conversation recorder service to persist conversation messages in real-time without disrupting ongoing calls.
- Update the frontend to display conversation history, including filtering and sorting options, improving user experience.
- Enhance the pipeline to integrate conversation history recording seamlessly during interactions.

backend/db/models.py

@@ -9,7 +9,17 @@
 
 from datetime import datetime
 
-from sqlalchemy import JSON, Boolean, DateTime, ForeignKey, String, func
+from sqlalchemy import (
+    JSON,
+    Boolean,
+    DateTime,
+    ForeignKey,
+    Integer,
+    String,
+    Text,
+    UniqueConstraint,
+    func,
+)
 from sqlalchemy.dialects.postgresql import JSONB
 from sqlalchemy.orm import DeclarativeBase, Mapped, mapped_column
 
@@ -187,3 +197,83 @@ class AssistantToolBinding(Base):
     created_at: Mapped[datetime] = mapped_column(
         DateTime(timezone=True), server_default=func.now()
     )
+
+
+class ConversationSession(Base):
+    """一次完整连接对应的对话会话；媒体文件以后作为 artifact 关联进来。"""
+
+    __tablename__ = "conversation_sessions"
+
+    id: Mapped[str] = mapped_column(String(40), primary_key=True)
+    assistant_id: Mapped[str | None] = mapped_column(
+        String(40),
+        ForeignKey("assistants.id", ondelete="SET NULL"),
+        nullable=True,
+        index=True,
+    )
+    assistant_name: Mapped[str] = mapped_column(String(128), default="")
+    channel: Mapped[str] = mapped_column(String(24), index=True)
+    runtime_mode: Mapped[str] = mapped_column(String(16), default="pipeline")
+    status: Mapped[str] = mapped_column(String(16), index=True, default="active")
+    message_count: Mapped[int] = mapped_column(Integer, default=0)
+    started_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), server_default=func.now(), index=True
+    )
+    ended_at: Mapped[datetime | None] = mapped_column(
+        DateTime(timezone=True), nullable=True
+    )
+    extra: Mapped[dict] = mapped_column(JSONB, default=dict)
+
+
+class ConversationMessage(Base):
+    """会话中的有序消息；当前只写 text，结构预留其他内容类型。"""
+
+    __tablename__ = "conversation_messages"
+    __table_args__ = (
+        UniqueConstraint(
+            "session_id", "sequence", name="uq_conversation_message_sequence"
+        ),
+    )
+
+    id: Mapped[str] = mapped_column(String(40), primary_key=True)
+    session_id: Mapped[str] = mapped_column(
+        String(40),
+        ForeignKey("conversation_sessions.id", ondelete="CASCADE"),
+        index=True,
+    )
+    sequence: Mapped[int] = mapped_column(Integer)
+    role: Mapped[str] = mapped_column(String(16))
+    content_type: Mapped[str] = mapped_column(String(16), default="text")
+    content: Mapped[str] = mapped_column(Text, default="")
+    occurred_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), server_default=func.now()
+    )
+    extra: Mapped[dict] = mapped_column(JSONB, default=dict)
+
+
+class ConversationArtifact(Base):
+    """未来的音频、视频、图片等大对象索引；二进制本体不进入数据库。"""
+
+    __tablename__ = "conversation_artifacts"
+
+    id: Mapped[str] = mapped_column(String(40), primary_key=True)
+    session_id: Mapped[str] = mapped_column(
+        String(40),
+        ForeignKey("conversation_sessions.id", ondelete="CASCADE"),
+        index=True,
+    )
+    message_id: Mapped[str | None] = mapped_column(
+        String(40),
+        ForeignKey("conversation_messages.id", ondelete="SET NULL"),
+        nullable=True,
+        index=True,
+    )
+    kind: Mapped[str] = mapped_column(String(16), index=True)
+    storage_uri: Mapped[str] = mapped_column(String(1024))
+    mime_type: Mapped[str] = mapped_column(String(128), default="")
+    size_bytes: Mapped[int | None] = mapped_column(Integer, nullable=True)
+    duration_ms: Mapped[int | None] = mapped_column(Integer, nullable=True)
+    created_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), server_default=func.now()
+    )
+    extra: Mapped[dict] = mapped_column(JSONB, default=dict)