Add Canton Participant Event Gateway proposal

Development Fund Proposal Submission

Proposal file: /proposals/canton-participant-event-gateway.md

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Summary

This PR has been reframed from Canton Event Indexer into Canton Participant Event Gateway: an open-source, participant-local application event delivery service for Canton dApps.

The revised proposal makes one clear architectural decision: direct Ledger API ingestion is the canonical source, and the funded core works end-to-end without PQS. PQS and Canton Index are treated as complementary infrastructure, not required dependencies.

The gateway focuses on the app integration layer that teams repeatedly rebuild:

• Daml interfaces as first-class event selectors, with template selectors as explicit opt-in/fallback.
• Participant-local Ledger API ingestion with durable checkpointing.
• REST/OpenAPI event history and replay.
• Reliable webhooks with HMAC signatures, retries, dead-letter queue, and replay.
• Party-scoped JWT/OIDC authorization.
• Docker/Helm packaging and reference integrations.
• 25% of the total request held for adoption validation.

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Checklist

• [x] Proposal file added under /proposals/
• [x] Milestones and funding amounts defined
• [x] Acceptance criteria included
• [x] Alignment with Canton priorities described

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Notes for Reviewers

• Funding request remains 400,000 CC, but the payment structure is now 120,000 / 120,000 / 60,000 / 100,000 CC, with the final 100,000 CC tied to external adoption validation.
• The proposal is no longer positioned as a PQS layer, PQS alternative, general query engine, GraphQL system, or typed analytical projection system.
• Requested primary SIG label: dapp-integration.
• canton-apis review may also be useful because the deliverable includes REST/OpenAPI and webhook APIs, but the proposal is intentionally scoped as application integration rather than base indexing/query infrastructure.
• Champion is still needed.