add architecture documentation and testing strategy

Co-authored-by: Copilot <copilot@github.com>

docs/4-solution-strategy.md

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+# 4. Solution Strategy
+
+A short summary and explanation of the fundamental decisions and solution strategies that shape system architecture. It includes:
+
+- Technology decisions
+- Decisions about the top-level decomposition of the system, e.g. usage of an architectural pattern or design pattern
+- Decisions on how to achieve key quality goals
+- Relevant organizational decisions, e.g. selecting a development process or delegating certain tasks to third parties
+
+Micro‑service style: a single FastAPI process handles all API endpoints; a separate Flask process serves the UI.
+
+| Quality Goal      | Scenario                 | Solution Approach                                         |
+| ----------------- | ------------------------ | --------------------------------------------------------- |
+| High availability | Service must be up 99.9% | Docker containers with health checks and restart policies |
+| Low latency       | Responses < 200 ms       | FastAPI async endpoints; DuckDB in-process                |
+| Data privacy      | Encrypted data           | JWT auth, HTTPS everywhere, DuckDB file encryption        |
+
+**Key technology decisions:**
+
+- **FastAPI** for async performance and automatic OpenAPI docs
+- **DuckDB** for a lightweight embedded DB with no separate server
+- **Flask** for a thin frontend layer with Jinja2 templates
+- **openrouter.ai** for unified access to multiple AI model providers