fix: use Grafast plan() instead of resolve() for downloadUrl field

[pyramation]

Summary

The downloadUrl computed field on @storageFiles-tagged file types was always returning null because it used a traditional GraphQL resolve() function. In PostGraphile V5, Grafast's planning system does not invoke traditional resolve() on PG table type fields — it only plans them as column lookups. Since downloadUrl is a computed field (not a real DB column), the resolver was silently never called.

This replaces resolve() with a Grafast plan() function using lambda(), object(), and grafastContext() — the same pattern already used by requestUploadUrl and confirmUpload mutations in this plugin. The underlying business logic (status check → per-database config resolution → public CDN URL or presigned GET URL) is unchanged.

Plugin version bumped from 0.1.0 → 0.2.0.

Review & Testing Checklist for Human

• Verify the Grafast plan pattern matches the mutation pattern in plugin.ts (~lines 217-403). The $parent.get('column') → object() → lambda() chain should mirror how requestUploadUrl accesses its input. This is the core correctness question.
• Verify context key names: the old code accessed context.withPgClient and context.pgSettings directly. The new code uses grafastContext().get('withPgClient') and grafastContext().get('pgSettings'). Confirm these keys exist in the Grafast context object (they should — the mutations use the same pattern).
• Confirm the conditional change is equivalent: old code used context.pgSettings ? context.withPgClient : null, new code uses if (withPgClient && pgSettings). Both guard against missing context, but the shape is slightly different.
• Test end-to-end with the companion test PR (constructive-db#934). That PR has 3 download tests that exercise this field via GraphQL: private file presigned URL, entity-member download, and public CDN URL. All 32 tests pass locally with MinIO. This repo's CI does not directly exercise the download path.

Notes

• There are no tests for the download URL field in this repo — coverage comes from the integration tests in constructive-db.
• All types in the plan function are any, consistent with the existing mutation patterns in this plugin. Proper typing would require importing Grafast step types.
• The silent catch {} fallback to global S3 config is preserved from the original code.

Link to Devin session: https://app.devin.ai/sessions/18879be982854a40abe5c9b915aa4a84
Requested by: @pyramation

[devin-ai-integration]

🤖 Devin AI Engineer

I'll be helping with this pull request! Here's what you should know:

✅ I will automatically:

• Address comments on this PR. Add '(aside)' to your comment to have me ignore it.
• Look at CI failures and help fix them

Note: I can only respond to comments from users who have write access to this repository.

⚙️ Control Options:

• Disable automatic comment and CI monitoring