perf(pipeline): parallel prefetch and lean revision GET

GetRevisionSummary без preview_fragments; boundedRepoCtx для GetRevision;
parallel CDN prefetch с ctx из ingest.

Co-authored-by: Cursor <[email protected]>

internal/repository/ctx.go

@@ -0,0 +1,19 @@
+package repository
+
+import (
+	"context"
+	"time"
+)
+
+const defaultRepoTimeout = 60 * time.Second
+
+// boundedRepoCtx returns a context with default repository I/O timeout.
+func boundedRepoCtx(parent context.Context) (context.Context, context.CancelFunc) {
+	if parent == nil {
+		parent = context.Background()
+	}
+	if _, ok := parent.Deadline(); ok {
+		return parent, func() {}
+	}
+	return context.WithTimeout(parent, defaultRepoTimeout)
+}