fix: GC has no grace period (TOCTOU with dedup uploads) (#86)

Fixes #71

## Why
`FindOrphanedBlobs` returned any blob not currently referenced. Because CAS dedups (the blob row can exist before its artifact/local_files row is written), a concurrent upload reusing an existing hash could have its S3 object deleted mid-flight by the GC.

## Changes
- `FindOrphanedBlobs` now takes a `minAge` and only returns blobs with `created_at < now()-minAge`.
- The collector passes a 1h `blobGracePeriod`.

## Validation
- `go test ./internal/gc/...` and `make e2e` pass.

---------

Co-authored-by: BenVincent <benvin@main.unkin.net>
Reviewed-on: #86
Co-authored-by: Ben Vincent <ben@unkin.net>
Co-committed-by: Ben Vincent <ben@unkin.net>

internal/gc/gc.go

@@ -9,6 +9,11 @@ import (
 	"git.unkin.net/unkin/artifactapi/internal/storage"
 )
 
+// blobGracePeriod is how old an orphaned blob must be before GC will delete
+// it. This avoids racing in-flight dedup uploads that insert the blob row
+// before the referencing artifact/local_files row exists.
+const blobGracePeriod = 1 * time.Hour
+
 type Collector struct {
 	db       *database.DB
 	store    *storage.S3
@@ -38,7 +43,7 @@ func (c *Collector) Run(ctx context.Context) {
 func (c *Collector) sweep(ctx context.Context) {
 	start := time.Now()
 
-	orphaned, err := c.db.FindOrphanedBlobs(ctx)
+	orphaned, err := c.db.FindOrphanedBlobs(ctx, blobGracePeriod)
 	if err != nil {
 		slog.Error("gc: find orphaned blobs", "error", err)
 		return