feat: implement data layer with comprehensive test infrastructure

- Define SQLModel schemas for Session, Note, and Link entities
  - Add API request/response models for RPC endpoints
  - Create LLM structured output models for Zettel extraction
  - Set up async database initialization with SQLModel and aiosqlite
  - Implement repository pattern for CRUD operations
  - Add complete test suite with pytest configuration
  - Create validation test runner for development workflow
  - Add .gitignore for Python/FastAPI project security

README.md

@@ -81,3 +81,60 @@ SkyTalkAPI: {
 # Connections to External APIs
 SkyTalkAPI.Services -> ExternalAPIs.Gemini
 ```
+
+---
+
+## MVP Features (Phase 1)
+
+At the completion of this implementation plan, the SkyTalk API will support the
+following core features:
+
+- **Session Management:** Start a new interview session based on an initial
+  topic.
+
+- **RAG-Powered Interviewing:** Engage in a back-and-forth conversation where
+  the AI's questions are informed by existing knowledge in the vector store.
+
+- **Automatic Session Termination:** The AI can detect a natural conclusion to
+  the conversation.
+
+- **Asynchronous Synthesis:** Once the interview ends, a background process is
+  triggered to analyze the transcript.
+
+- **Semantic Segmentation:** The transcript is intelligently broken down into
+  atomic "Zettels" (notes), each focusing on a single concept.
+
+- **Vector Indexing:** Each new note is converted into a vector embedding and
+  stored for future RAG.
+
+- **Generative Linking:** The system identifies semantically related notes and
+  uses an LLM to generate a rich, contextual link explaining the relationship
+  between them.
+
+- **Status Tracking:** Endpoints to check the status of a session (active,
+  processing, completed).
+
+---
+
+## Next Steps (Post-MVP)
+
+- **Authentication:** Implement user accounts and authentication (e.g., JWT) to
+  create user-specific knowledge bases.
+
+- **Frontend Integration:** Build a web-based frontend (e.g., at
+  www.skytalk.app) that consumes this API.
+
+- **Knowledge Graph Visualization:** Add endpoints to export the note-and-link
+  structure in a format suitable for graph visualization libraries (e.g., D3.js,
+  Vis.js).
+
+- **Note Editing and Management:** Provide endpoints for users to manually edit,
+  delete, or merge notes.
+
+- **Advanced Search:** Implement more sophisticated search functionalities
+  beyond simple semantic search, such as filtering by tags or searching within
+  link contexts.
+
+- **Scalable Infrastructure:** Migrate from SQLite/embedded ChromaDB to a
+  production-grade database (e.g., PostgreSQL with pgvector) and a managed
+  vector database for scalability.