feat(api): add chat functionality (Sprint 3)

Implement Sprint 3: Chat Functionality

- Add ChatService with send_message and get_history methods
- Add POST /api/v1/notebooks/{id}/chat - Send message
- Add GET /api/v1/notebooks/{id}/chat/history - Get chat history
- Add ChatRequest model (message, include_references)
- Add ChatResponse model (message, sources[], timestamp)
- Add ChatMessage model (id, role, content, timestamp, sources)
- Add SourceReference model (source_id, title, snippet)
- Integrate chat router with main app

Features:
- Send messages to notebook chat
- Get AI responses with source references
- Retrieve chat history
- Support for citations in responses

Tests:
- 14 unit tests for ChatService
- 11 integration tests for chat API
- 25/25 tests passing

Related: Sprint 3 - Chat Functionality

src/notebooklm_agent/api/models/responses.py

@@ -418,3 +418,129 @@ class HealthStatus(BaseModel):
         description="Service name",
         examples=["notebooklm-agent-api"],
     )
+
+
+class SourceReference(BaseModel):
+    """Source reference in chat response.
+
+    Attributes:
+        source_id: The source ID.
+        title: The source title.
+        snippet: Relevant text snippet from the source.
+    """
+
+    model_config = ConfigDict(
+        json_schema_extra={
+            "example": {
+                "source_id": "550e8400-e29b-41d4-a716-446655440001",
+                "title": "Example Article",
+                "snippet": "Key information from the source...",
+            }
+        }
+    )
+
+    source_id: str = Field(
+        ...,
+        description="The source ID",
+        examples=["550e8400-e29b-41d4-a716-446655440001"],
+    )
+    title: str = Field(
+        ...,
+        description="The source title",
+        examples=["Example Article"],
+    )
+    snippet: str | None = Field(
+        None,
+        description="Relevant text snippet from the source",
+        examples=["Key information from the source..."],
+    )
+
+
+class ChatResponse(BaseModel):
+    """Chat response model.
+
+    Attributes:
+        message: The assistant's response message.
+        sources: List of source references used in the response.
+        timestamp: Response timestamp.
+    """
+
+    model_config = ConfigDict(
+        json_schema_extra={
+            "example": {
+                "message": "Based on the sources, here are the key points...",
+                "sources": [
+                    {
+                        "source_id": "550e8400-e29b-41d4-a716-446655440001",
+                        "title": "Example Article",
+                        "snippet": "Key information...",
+                    }
+                ],
+                "timestamp": "2026-04-06T10:30:00Z",
+            }
+        }
+    )
+
+    message: str = Field(
+        ...,
+        description="The assistant's response message",
+        examples=["Based on the sources, here are the key points..."],
+    )
+    sources: list[SourceReference] = Field(
+        default_factory=list,
+        description="List of source references used in the response",
+    )
+    timestamp: datetime = Field(
+        ...,
+        description="Response timestamp",
+        examples=["2026-04-06T10:30:00Z"],
+    )
+
+
+class ChatMessage(BaseModel):
+    """Chat message model.
+
+    Attributes:
+        id: Unique message identifier.
+        role: Message role (user or assistant).
+        content: Message content.
+        timestamp: Message timestamp.
+        sources: Source references (for assistant messages).
+    """
+
+    model_config = ConfigDict(
+        json_schema_extra={
+            "example": {
+                "id": "550e8400-e29b-41d4-a716-446655440002",
+                "role": "assistant",
+                "content": "Based on the sources...",
+                "timestamp": "2026-04-06T10:30:00Z",
+                "sources": [],
+            }
+        }
+    )
+
+    id: str = Field(
+        ...,
+        description="Unique message identifier",
+        examples=["550e8400-e29b-41d4-a716-446655440002"],
+    )
+    role: str = Field(
+        ...,
+        description="Message role (user or assistant)",
+        examples=["user", "assistant"],
+    )
+    content: str = Field(
+        ...,
+        description="Message content",
+        examples=["What are the key points?"],
+    )
+    timestamp: datetime = Field(
+        ...,
+        description="Message timestamp",
+        examples=["2026-04-06T10:30:00Z"],
+    )
+    sources: list[SourceReference] | None = Field(
+        None,
+        description="Source references (for assistant messages)",
+    )