[W5][BE][M] Add partition-ready event storage key strategy notes and guardrails

[kike-alt]

ID: W5-3C-BE-003
Track: BE
Phase: 3C
Difficulty: intermediate
Priority: P2

Description

Prepare storage model for scaling event volume with partition-friendly key semantics.

Acceptance Criteria

• Define implementation plan aligned to scope boundaries
• Implement and verify changes in backend/src/indexer/entities
• Implement and verify changes in docs/BACKEND_INDEXER_FOUNDATION.md
• Add/adjust tests or validation artifacts for the changed surface
• Update docs/notes if behavior or operations change

Relevant Files/Modules

• backend/src/indexer/entities
• docs/BACKEND_INDEXER_FOUNDATION.md

Dependencies / Blocked By

• [W5][BE][M] Implement Soroban RPC event poller scaffold into indexer service #593

Validation Requirements

• design notes and migration guard tests reviewed