Create docs/architecture/ with diagrams

[osvaldoandrade]

Add visual architecture documentation.

Goal: C4 and sequence diagrams for key flows.

Implementation:

• Create docs/architecture/
• Add C4 context diagram (system level)
• Add C4 container diagram (services/datastores)
• Add sequence diagrams:
  • Task creation flow (producer → server → storage)
  • Task claim flow (worker → server → storage → lease)
  • Result delivery flow (worker → server → webhook)
• Use Mermaid (text-based, version controllable)

Acceptance criteria:

• Diagrams render correctly on GitHub
• Key flows documented
• Images ≥1600×900 with captions

Part of: #655