deploy: use dedicated ClickHouse migrator image

[batond]

Summary

• run the ClickHouse migration job from the dedicated migrator image in Helm and Docker Compose
• keep the Postgres migrator on the Enterprise app image
• add independent Helm values and a CLICKHOUSE_MIGRATOR_IMAGE Compose override
• update the image bump script so future app SHA bumps also select <sha>-clickhouse-migrator
• update upgrade and troubleshooting documentation

Image

ghcr.io/git-ai-project/git-ai-web-ee:7160143-clickhouse-migrator

This immutable tag is published by next-element-inc/monorepo#674 from the existing GHCR package. The image defines the migration command itself, so explicit command overrides in Helm and Compose are no longer needed.

Validation

• Helm lint passes
• rendered the ClickHouse Job with dependencies and confirmed the dedicated image, environment, and resources
• Docker Compose config parses and resolves the dedicated image
• image bump script passes bash -n
• the image ran all 34 migrations against ClickHouse 25.8.9.20, skipped all 34 on a second run, and rejected a changed applied checksum

Rollout order

Merge after monorepo#674 has published the tag and before monorepo#675 removes the ClickHouse payload from the app image.