[W5][BE][M] Add dead-letter queue scaffold for permanently invalid events

[kike-alt]

ID: W5-3C-BE-004
Track: BE
Phase: 3C
Difficulty: intermediate
Priority: P1

Description

Route unrecoverable invalid events to dead-letter storage for auditability.

Acceptance Criteria

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

Relevant Files/Modules

• backend/src/indexer
• backend/src/indexer/entities

Dependencies / Blocked By

• [W5][BE][M] Introduce normalized event schema validation at ingest boundary #596
• [W5][SECURITY][S] Add backend ingest payload size and topic allowlist guards #604

Validation Requirements

• invalid fixture lands in dead-letter path with metadata