Add safe CSV artifact preview to job detail UI

Adds a Preview action next to eligible partial/final CSV artifacts on
the job detail page. Reads at most 64 KiB and 30 rows via a coordinator-
owned blob open, verifying job ownership and the same downloadable rule
as the existing download proxy so an artifact ID from another job is
never disclosed. Non-CSV and malformed/empty content fail safely with a
sanitized message instead of being rendered as text; all cell values go
through html/template escaping.

coordinator/internal/usecase/preview.go

@@ -0,0 +1,150 @@
+package usecase
+
+import (
+	"context"
+	"encoding/csv"
+	"errors"
+	"io"
+	"strings"
+
+	"github.com/google/uuid"
+
+	"github.com/emil28092005/SciMesh/coordinator/internal/domain"
+)
+
+// previewMaxRows and previewMaxBytes bound how much of an artifact the
+// diagnostic preview ever reads or renders: a partial shard CSV can be large,
+// and this is a diagnostic aid, not a viewer for the full file.
+const (
+	previewMaxRows  = 30
+	previewMaxBytes = 64 * 1024
+)
+
+// ArtifactPreviewView is what the UI renders for a diagnostic CSV preview. It
+// never carries a storage path, database error, or worker-local detail.
+type ArtifactPreviewView struct {
+	JobID       string
+	ArtifactID  string
+	Filename    string
+	Previewable bool
+	Reason      string
+	Headers     []string
+	Rows        [][]string
+	Truncated   bool
+	RowLimit    int
+	ByteLimit   int64
+}
+
+// PreviewArtifact renders at most the first previewMaxRows rows of a CSV
+// artifact, reading at most previewMaxBytes from storage. It reuses the same
+// job-scoped, downloadable-artifact rule as the download proxy so an artifact
+// ID from another job is never previewable.
+type PreviewArtifact struct {
+	read  UIReadRepository
+	blobs BlobStore
+}
+
+func NewPreviewArtifact(read UIReadRepository, blobs BlobStore) *PreviewArtifact {
+	return &PreviewArtifact{read: read, blobs: blobs}
+}
+
+func (p *PreviewArtifact) Execute(ctx context.Context, jobID, artifactID uuid.UUID) (ArtifactPreviewView, error) {
+	job, err := p.read.GetJob(ctx, jobID)
+	if err != nil {
+		return ArtifactPreviewView{}, err
+	}
+	tasks, err := p.read.ListTasksByJob(ctx, jobID)
+	if err != nil {
+		return ArtifactPreviewView{}, err
+	}
+	// Same status derivation the dashboard uses, so a final artifact previews
+	// exactly when it would also be offered for download.
+	status := jobCard(*job, tasks).Status
+
+	artifacts, err := p.read.ListArtifactsByJob(ctx, jobID)
+	if err != nil {
+		return ArtifactPreviewView{}, err
+	}
+	var art *domain.Artifact
+	for i := range artifacts {
+		if artifacts[i].ID == artifactID {
+			art = &artifacts[i]
+			break
+		}
+	}
+	if art == nil {
+		return ArtifactPreviewView{}, domain.ErrArtifactNotFound
+	}
+	downloadable := art.Kind == domain.ArtifactPartialResult ||
+		(art.Kind == domain.ArtifactFinalResult && status == string(domain.JobCompleted))
+	if !downloadable {
+		return ArtifactPreviewView{}, domain.ErrArtifactNotFound
+	}
+
+	view := ArtifactPreviewView{
+		JobID:      jobID.String(),
+		ArtifactID: art.ID.String(),
+		Filename:   art.Filename,
+		RowLimit:   previewMaxRows,
+		ByteLimit:  previewMaxBytes,
+	}
+	if !isCSVArtifact(art) {
+		view.Reason = "This artifact is not a CSV file, so it cannot be shown as text here. Download it instead."
+		return view, nil
+	}
+	if art.SizeBytes == 0 {
+		view.Reason = "This artifact is empty."
+		return view, nil
+	}
+
+	rc, err := p.blobs.Open(ctx, art.StorageKey)
+	if err != nil {
+		return ArtifactPreviewView{}, err
+	}
+	defer func() { _ = rc.Close() }()
+
+	// LimitedReader caps the bytes read from storage regardless of how many
+	// rows are found within that window — the artifact is never loaded whole.
+	limited := &io.LimitedReader{R: rc, N: previewMaxBytes}
+	reader := csv.NewReader(limited)
+	reader.FieldsPerRecord = -1 // a byte-limited cut mid-row must not look like a schema error
+
+	header, err := reader.Read()
+	if err != nil {
+		view.Reason = "This artifact could not be read as CSV."
+		return view, nil
+	}
+	view.Headers = append([]string(nil), header...)
+
+	rows := make([][]string, 0, previewMaxRows)
+	for len(rows) < previewMaxRows {
+		record, err := reader.Read()
+		if err != nil {
+			if !errors.Is(err, io.EOF) {
+				// Malformed content further into the stream: keep what parsed
+				// cleanly and say the preview stopped early.
+				view.Truncated = true
+			}
+			break
+		}
+		rows = append(rows, append([]string(nil), record...))
+	}
+	view.Rows = rows
+
+	if art.SizeBytes > previewMaxBytes {
+		view.Truncated = true
+	} else if len(rows) == previewMaxRows {
+		if _, err := reader.Read(); err == nil {
+			view.Truncated = true
+		}
+	}
+	view.Previewable = true
+	return view, nil
+}
+
+func isCSVArtifact(a *domain.Artifact) bool {
+	if a.ContentType == "text/csv" {
+		return true
+	}
+	return strings.HasSuffix(strings.ToLower(a.Filename), ".csv")
+}