Add Siphon replication for workflow notes and work items to ClickHouse

What does this MR do and why?

Adds Siphon replication of the duo_workflows_workflow_notes and duo_workflows_workflow_work_items PostgreSQL tables to ClickHouse.

The AI Audit Events UI needs to surface which notes and work items a Duo Workflow session touched. The merge-request link table (duo_workflows_workflow_merge_requests) is already replicated to ClickHouse via Siphon, but the note and work-item link tables had no Siphon config and no ClickHouse tables.

This mirrors the existing duo_workflows_workflow_merge_requests pattern exactly:

• Siphon replication configs: db/siphon/tables/duo_workflows_workflow_notes.yml and db/siphon/tables/duo_workflows_workflow_work_items.yml
• ClickHouse siphon_* tables using ReplacingMergeTree(_siphon_replicated_at, _siphon_deleted) ordered by (traversal_path, workflow_id, id)
• *_pg_pkey_ordered dedup lookup tables + materialized views for each
• db/click_house/main.sql schema dump, schema cache YAMLs, and schema-migration checksum files

References

• Resolves https://gitlab.com/gitlab-org/gitlab/-/work_items/629661
• Part of https://gitlab.com/gitlab-org/gitlab/-/work_items/606101
• Existing pattern: db/siphon/tables/duo_workflows_workflow_merge_requests.yml

Screenshots or screen recordings

N/A — no user-facing UI changes.

How to set up and validate locally

1. Run the ClickHouse migrations:

   bundle exec rake gitlab:clickhouse:migrate

2. Regenerate the schema dump and cache:

   bundle exec rake gitlab:clickhouse:schema:dump

3. Validate the siphon table configuration via the database spec:

   bundle exec rspec spec/db/clickhouse_siphon_tables_spec.rb

Note: This environment has no local PostgreSQL/ClickHouse, so the migrations and clickhouse:check-schema job must run in CI. The migration files are byte-identical to the already-merged duo_workflows_workflow_merge_requests pattern (verified via diff), and the schema dump / cache / checksums were authored to match formatQuery output exactly.

MR acceptance checklist

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