chore: sequin documentation

[epipav]

Note

Low Risk
Documentation-only change with no impact on production code paths.

Overview
Adds a new services/libs/tinybird/sequin.md architecture document describing Sequin’s PostgreSQL CDC pipeline end-to-end (logical replication/WAL concepts, runtime components and message flow, backfills, buffering/backpressure, persistence/idempotency, and ops/debugging guidance).

No application code or runtime behavior changes; this PR is documentation-only.

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[joanagmaia]

LGTM 🚀