feat(store): add blob, file, folder, and upload stores

Add BlobStore (ref counting), FileStore (soft delete + pagination), FolderStore (materialized path), UploadStore (idempotent upsert), and update AutoMigrate.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

internal/store/blob_store.go

@@ -0,0 +1,99 @@
+package store
+
+import (
+	"context"
+	"errors"
+
+	"gcy_hpc_server/internal/model"
+
+	"gorm.io/gorm"
+	"gorm.io/gorm/clause"
+)
+
+// BlobStore manages physical file blobs with reference counting.
+type BlobStore struct {
+	db *gorm.DB
+}
+
+// NewBlobStore creates a new BlobStore.
+func NewBlobStore(db *gorm.DB) *BlobStore {
+	return &BlobStore{db: db}
+}
+
+// Create inserts a new FileBlob record.
+func (s *BlobStore) Create(ctx context.Context, blob *model.FileBlob) error {
+	return s.db.WithContext(ctx).Create(blob).Error
+}
+
+// GetBySHA256 returns the FileBlob with the given SHA256 hash.
+// Returns (nil, nil) if not found.
+func (s *BlobStore) GetBySHA256(ctx context.Context, sha256 string) (*model.FileBlob, error) {
+	var blob model.FileBlob
+	err := s.db.WithContext(ctx).Where("sha256 = ?", sha256).First(&blob).Error
+	if errors.Is(err, gorm.ErrRecordNotFound) {
+		return nil, nil
+	}
+	if err != nil {
+		return nil, err
+	}
+	return &blob, nil
+}
+
+// IncrementRef atomically increments the ref_count for the blob with the given SHA256.
+func (s *BlobStore) IncrementRef(ctx context.Context, sha256 string) error {
+	result := s.db.WithContext(ctx).Model(&model.FileBlob{}).
+		Where("sha256 = ?", sha256).
+		UpdateColumn("ref_count", gorm.Expr("ref_count + 1"))
+	if result.Error != nil {
+		return result.Error
+	}
+	if result.RowsAffected == 0 {
+		return gorm.ErrRecordNotFound
+	}
+	return nil
+}
+
+// DecrementRef atomically decrements the ref_count for the blob with the given SHA256.
+// Returns the new ref_count after decrementing.
+func (s *BlobStore) DecrementRef(ctx context.Context, sha256 string) (int64, error) {
+	result := s.db.WithContext(ctx).Model(&model.FileBlob{}).
+		Where("sha256 = ? AND ref_count > 0", sha256).
+		UpdateColumn("ref_count", gorm.Expr("ref_count - 1"))
+	if result.Error != nil {
+		return 0, result.Error
+	}
+	if result.RowsAffected == 0 {
+		return 0, gorm.ErrRecordNotFound
+	}
+
+	var blob model.FileBlob
+	if err := s.db.WithContext(ctx).Where("sha256 = ?", sha256).First(&blob).Error; err != nil {
+		return 0, err
+	}
+	return int64(blob.RefCount), nil
+}
+
+// Delete removes a FileBlob record by SHA256 (hard delete).
+func (s *BlobStore) Delete(ctx context.Context, sha256 string) error {
+	result := s.db.WithContext(ctx).Where("sha256 = ?", sha256).Delete(&model.FileBlob{})
+	if result.Error != nil {
+		return result.Error
+	}
+	return nil
+}
+
+// GetBySHA256ForUpdate returns the FileBlob with a SELECT ... FOR UPDATE lock.
+// Returns (nil, nil) if not found.
+func (s *BlobStore) GetBySHA256ForUpdate(ctx context.Context, tx *gorm.DB, sha256 string) (*model.FileBlob, error) {
+	var blob model.FileBlob
+	err := tx.WithContext(ctx).
+		Clauses(clause.Locking{Strength: "UPDATE"}).
+		Where("sha256 = ?", sha256).First(&blob).Error
+	if errors.Is(err, gorm.ErrRecordNotFound) {
+		return nil, nil
+	}
+	if err != nil {
+		return nil, err
+	}
+	return &blob, nil
+}