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/store/backend.go

@@ -76,6 +76,8 @@ type Backend interface {
 	DeleteSpeaker(tenantID, id string) error
 
 	GetRevision(tenantID, revisionID string) (*Revision, error)
+	// GetRevisionSummary returns revision metadata without preview_fragments payloads.
+	GetRevisionSummary(tenantID, revisionID string) (*Revision, error)
 	ListRevisions(tenantID, moduleID string, cursor string, limit int) (items []*Revision, nextCursor string, hasMore bool)
 	ListRevisionPrefixes(tenantID, revisionID string, cursor string, limit int) (prefixes []PrefixRow, next string, more bool)
 	CreateRollbackRevision(tenantID, sourceRevisionID string) (newID string, err error)

internal/store/memory.go

@@ -424,14 +424,19 @@ func (m *Memory) GetModule(tenantID, moduleID string) (*Module, error) {
 func (m *Memory) GetRevision(tenantID, revisionID string) (*Revision, error) {
 	m.mu.RLock()
 	defer m.mu.RUnlock()
-	rev, ok := m.revisions[revisionID]
-	if !ok {
-		return nil, ErrNotFound
+	return m.getRevisionLocked(tenantID, revisionID)
+}
+
+func (m *Memory) GetRevisionSummary(tenantID, revisionID string) (*Revision, error) {
+	m.mu.RLock()
+	defer m.mu.RUnlock()
+	rev, err := m.getRevisionLocked(tenantID, revisionID)
+	if err != nil {
+		return nil, err
 	}
-	if rev.TenantID != tenantID {
-		return nil, ErrTenantScope
-	}
-	return rev, nil
+	cp := *rev
+	cp.PreviewFragments = nil
+	return &cp, nil
 }
 
 func (m *Memory) GetSpeaker(tenantID, speakerID string) (*Speaker, error) {