Implement db-v tag database export release workflow

[jmankoff]

We want a dedicated workflow for database export releases triggered by db-v tags, without changing app image release behavior.

• Add a GitHub Actions workflow triggered by push tags matching db-v* and manual dispatch.
• Need to decide what is ok to release from the database and confirm that's all we're releasing.
• Export the intended database artifacts in a repeatable way.
• Publish artifacts as release assets with clear version naming.
• Keep this isolated from app image publishing in docker workflow.

Proposed setup:

• push tags: db-v*
• use dedicated production-db-export environment
• require reviewer approval for publish path
• use test and production Supabase secrets scoped to job needs
• release tags based on db-vX.Y.Z input tag
• assets include scope in filename for easy consumption

Output artifacts:

• public dataset SQL (again, need to decide what goes here and how to properly eliminate hidden data)
• test seed SQL
• optional full private export SQL only when explicitly requested

Documentation

• Update release guidance in README
• Add operational runbook for DB exports in documentation folder.