refactor: reorganize folder structure for better maintainability

Changes:
- Move migrations/ under open_notebook/database/migrations/
- Extract AI models to open_notebook/ai/ (Model, ModelManager, provision)
- Extract podcasts to open_notebook/podcasts/ (EpisodeProfile, SpeakerProfile, PodcastEpisode)
- Reorganize prompts to mirror graphs structure (chat/, source_chat/)

This improves code organization by:
- Consolidating database concerns (migrations now with database code)
- Separating AI infrastructure from domain entities
- Isolating podcast feature into its own module
- Creating consistent prompt/graph naming conventions

All 52 tests pass.

open_notebook/graphs/source_chat.py

@@ -5,16 +5,15 @@ from typing import Annotated, Dict, List, Optional
 from ai_prompter import Prompter
 from langchain_core.messages import AIMessage, SystemMessage
 from langchain_core.runnables import RunnableConfig
-
-from open_notebook.utils import clean_thinking_content
 from langgraph.checkpoint.sqlite import SqliteSaver
 from langgraph.graph import END, START, StateGraph
 from langgraph.graph.message import add_messages
 from typing_extensions import TypedDict
 
+from open_notebook.ai.provision import provision_langchain_model
 from open_notebook.config import LANGGRAPH_CHECKPOINT_FILE
 from open_notebook.domain.notebook import Source, SourceInsight
-from open_notebook.graphs.utils import provision_langchain_model
+from open_notebook.utils import clean_thinking_content
 from open_notebook.utils.context_builder import ContextBuilder
 
 
@@ -111,7 +110,7 @@ def call_model_with_source_context(
     }
 
     # Apply the source_chat prompt template
-    system_prompt = Prompter(prompt_template="source_chat").render(data=prompt_data)
+    system_prompt = Prompter(prompt_template="source_chat/system").render(data=prompt_data)
     payload = [SystemMessage(content=system_prompt)] + state.get("messages", [])
 
     # Handle async model provisioning from sync context