docs: Architecture deep-dive & data-flow diagrams refresh

[joelpeace48-cell]

Background & current state

docs/ARCHITECTURE_OVERVIEW.md + docs/FLOWS.md predate the new subsystems (indexer, ZK, payouts, notifications, multi-tenancy). Diagrams will be stale as these land.

Goal

Refresh the architecture deep-dive + data-flow diagrams to include the new subsystems, keeping the canonical system map accurate.

Outline

• Update component diagram + trust boundaries; add data-flow diagrams for: indexed read path, ZK registration, asset payout/claim, notifications, multi-tenant request scoping.

Task breakdown

• Update ARCHITECTURE_OVERVIEW.md + FLOWS.md diagrams.
• Add per-subsystem flow diagrams.
• Review vs code; link from README.

Acceptance criteria

• Diagrams match the current architecture incl. new subsystems.

Verification

• Maintainer review vs implementation.

Difficulty: medium · Effort: M · documentation