feat(indexer): expose lag and finality SLOs (#483)

[buki70]

Linked task

Closes:
Head SHA reviewed: <!-- full SHA -->

Summary

Scope and assignment

• The linked issue has the exact Stellar Wave label.
• The PR author is assigned or explicitly approved by a maintainer.
• This PR resolves one task and all dependencies are safely completed.

Architecture and security

• Contracts/finalized events remain authoritative for protocol state.
• No backend-authoritative claim, vote, dispute, settlement, reward, treasury, or governance mutation was added.
• TypeORM/PostgreSQL remains the only persistence architecture; Prisma was not introduced.
• Reorg, duplicate delivery, retry, finality, stale-cache, and degraded-dependency behavior is fail-closed.
• SIWE/authentication, authorization, validation, redaction, and rate-limit impact was reviewed.
• No Stellar/Soroban/Freighter runtime, secret, placeholder production value, or production mock is included.

Validation

• Lint, typecheck, and build pass.
• Unit and PostgreSQL integration tests pass.
• Migrations and rollback validation pass.
• Indexer/reorg and protocol-invariant tests pass.
• Security, container, and artifact-drift checks pass.
• Required human CODEOWNER approval applies to this exact head SHA.

closes #483

[coderabbitai]

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