Add comprehensive architecture documentation

[NikolayS]

Summary

This PR adds a new ARCHITECTURE.md file that documents the postgresai system design through two complementary views: a high-level data-flow diagram and a detailed component diagram. Both diagrams use Mermaid syntax for easy rendering on GitHub/GitLab.

Key Changes

• High-level view: Illustrates the main actors (User, CLI, Target PostgreSQL, Monitoring Stack, Cloud), the two storage backends (Postgres sink and VictoriaMetrics), and how data flows between them
• Component view: Expands the system to show individual containers/services including:
  • User-side tools (CLI, AI IDE, MCP server)
  • Target databases (PostgreSQL 14–18 with extensions)
  • Monitoring stack services (pgwatch, Grafana, Flask backend, reporter, self-monitoring)
  • Cloud platform (console.postgres.ai)
• Component cheat sheet: Reference table explaining the role of each major component
• Storage rationale: Explains why both VictoriaMetrics (time-series) and Postgres sink (structured rows) are used

Notable Details

• Both diagrams use Mermaid flowchart syntax with color-coded styling for different system layers (storage, cloud, user, targets)
• Documentation emphasizes key architectural decisions: dual-storage approach, optional cloud integration, and the CLI as the unified entry point
• Includes connection details (protocols, data types) on diagram edges for clarity
• Covers both express checkup (one-shot) and full monitoring (continuous) modes

https://claude.ai/code/session_015msHQaT5kpRiSCqL6frw7i