feat(db): add chat and message models for conversation tracking

Introduce SQLModel-based Chat and Message models to store conversation history. The Chat model tracks user conversations while the Message model stores individual messages with metadata including role, content, answer, and tool-related fields.

.gitattributes

@@ -33,3 +33,4 @@ saved_model/**/* filter=lfs diff=lfs merge=lfs -text
 *.zip filter=lfs diff=lfs merge=lfs -text
 *.zst filter=lfs diff=lfs merge=lfs -text
 *tfevents* filter=lfs diff=lfs merge=lfs -text
+*.png filter=lfs diff=lfs merge=lfs -text

db/models/chat.py

@@ -0,0 +1,22 @@
+import uuid
+from datetime import datetime
+from sqlmodel import Field, Relationship, SQLModel
+from typing import TYPE_CHECKING, list
+
+if TYPE_CHECKING:
+    from .message import Message
+
+class Chat(SQLModel, table=True):
+    id: uuid.UUID = Field(default_factory=uuid.uuid4, primary_key=True, index=True)
+    user_id: uuid.UUID = Field(index=True)
+    title: str = Field(default="New Chat", max_length=100)
+    created_at: datetime = Field(default_factory=datetime.utcnow, nullable=False)
+    updated_at: datetime = Field(
+        default_factory=datetime.utcnow, 
+        nullable=False, 
+        sa_column_kwargs={"onupdate": datetime.utcnow}
+    )
+    messages: list["Message"] = Relationship(
+        back_populates="chat",
+        sa_relationship_kwargs={"cascade": "all, delete-orphan"}
+    )

db/models/message.py

@@ -0,0 +1,33 @@
+import uuid
+from datetime import datetime
+from typing import Optional, Literal, Any, TYPE_CHECKING
+from sqlmodel import Field, Relationship, SQLModel, JSON, Column
+
+if TYPE_CHECKING:
+    from .chat import Chat
+
+MessageRole = Literal["user", "assistant", "tool"]
+
+class Message(SQLModel, table=True):
+    id: uuid.UUID = Field(default_factory=uuid.uuid4, primary_key=True)
+    chat_id: uuid.UUID = Field(foreign_key="chat.id", index=True)
+    role: MessageRole = Field(nullable=False)
+
+    # Stores the RAW, full text content.
+    # e.g., "The answer is... Links: [http://...]"
+    content: Optional[str] = Field(default=None, sa_column_kwargs={"longtext": True})
+
+    # Stores the CLEAN, parsed answer for display on the frontend.
+    # Populated only for the final AI message of a turn.
+    answer: Optional[str] = Field(default=None, sa_column_kwargs={"longtext": True})
+
+    # Stores the parsed source links for the frontend.
+    links: Optional[list[str]] = Field(default=None, sa_column=Column(JSON))
+
+    # Fields for tool-calling content
+    tool_calls: Optional[list[dict[str, Any]]] = Field(default=None, sa_column=Column(JSON))
+    tool_call_id: Optional[str] = Field(default=None, index=True)
+
+    created_at: datetime = Field(default_factory=datetime.utcnow, nullable=False)
+
+    chat: Optional["Chat"] = Relationship(back_populates="messages")