Add sequence diagrams and architecture documentation

[Copilot]

Added visual documentation for system flows and architecture using Mermaid diagrams.

Files Added

• SEQUENCE_DIAGRAMS.md - 8 sequence diagrams covering authentication, monitor lifecycle, health checks, incidents, and alerts
• ARCHITECTURE.md - System architecture, state machines, ERD, worker flows, and scalability patterns
• DOCUMENTATION_INDEX.md - Navigation guide with use-case-based access patterns

Coverage

Sequence Diagrams:

• Authentication (register/login)
• Monitor CRUD and lifecycle (start/pause/resume/update)
• Health check execution with 3-retry backoff
• Incident management (auto-create, acknowledge, resolve)
• Alert dispatch (webhook, email, Slack, Discord)

Architecture:

• Component interaction topology
• Database schema with 6 entities
• State machines for Monitor (PENDING → UP/DOWN/PAUSED) and Incident (OPEN → ACKNOWLEDGED → RESOLVED)
• Worker flowcharts showing retry logic and failure handling
• Horizontal scaling architecture

All diagrams use Mermaid for GitHub native rendering.

Original prompt

give me sequence diagram for this proejct . check it in detaols

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