feat(kc): checkout packing + ETag caching (Phase 2)

Implements issue #142 — .kc checkout pipeline that repacks silo/ entries
with current DB state before serving downloads.

When a client downloads a .kc file via GET /api/items/{pn}/file/{rev},
the server now:
1. Reads the file from storage into memory
2. Checks for silo/ directory (plain .fcstd files bypass packing)
3. Repacks silo/ entries with current item_metadata + revision history
4. Streams the repacked ZIP to the client

New files:
- internal/kc/pack.go: Pack() replaces silo/ entries in ZIP, preserving
  all non-silo entries (FreeCAD files, thumbnails) with original
  compression and timestamps. HasSiloDir() for lightweight detection.
- internal/api/pack_handlers.go: packKCFile server helper, computeETag,
  canSkipRepack lazy optimization.

ETag caching:
- ETag computed from revision_number + metadata.updated_at
- If-None-Match support returns 304 Not Modified before reading storage
- Cache-Control: private, must-revalidate

Lazy packing optimization:
- Skips repack if revision_hash matches and metadata unchanged since upload

Phase 2 packs: manifest.json, metadata.json, history.json,
dependencies.json (empty []). Approvals, macros, jobs deferred to
Phase 3-5.

Closes #142

internal/api/pack_handlers.go

@@ -0,0 +1,97 @@
+package api
+
+import (
+	"context"
+	"crypto/sha256"
+	"encoding/hex"
+	"fmt"
+	"time"
+
+	"github.com/kindredsystems/silo/internal/db"
+	"github.com/kindredsystems/silo/internal/kc"
+)
+
+// packKCFile gathers DB state and repacks silo/ entries in a .kc file.
+func (s *Server) packKCFile(ctx context.Context, data []byte, item *db.Item, rev *db.Revision, meta *db.ItemMetadata) ([]byte, error) {
+	manifest := &kc.Manifest{
+		UUID:         item.ID,
+		KCVersion:    derefStr(meta.KCVersion, "1.0"),
+		RevisionHash: derefStr(meta.RevisionHash, ""),
+		SiloInstance: derefStr(meta.SiloInstance, ""),
+	}
+
+	metadata := &kc.Metadata{
+		SchemaName:     derefStr(meta.SchemaName, ""),
+		Tags:           meta.Tags,
+		LifecycleState: meta.LifecycleState,
+		Fields:         meta.Fields,
+	}
+
+	// Build history from last 20 revisions.
+	revisions, err := s.items.GetRevisions(ctx, item.ID)
+	if err != nil {
+		return nil, fmt.Errorf("getting revisions: %w", err)
+	}
+	limit := 20
+	if len(revisions) < limit {
+		limit = len(revisions)
+	}
+	history := make([]kc.HistoryEntry, limit)
+	for i, r := range revisions[:limit] {
+		labels := r.Labels
+		if labels == nil {
+			labels = []string{}
+		}
+		history[i] = kc.HistoryEntry{
+			RevisionNumber: r.RevisionNumber,
+			CreatedAt:      r.CreatedAt.UTC().Format(time.RFC3339),
+			CreatedBy:      r.CreatedBy,
+			Comment:        r.Comment,
+			Status:         r.Status,
+			Labels:         labels,
+		}
+	}
+
+	input := &kc.PackInput{
+		Manifest:     manifest,
+		Metadata:     metadata,
+		History:      history,
+		Dependencies: []any{}, // empty for Phase 2
+	}
+
+	return kc.Pack(data, input)
+}
+
+// computeETag generates a quoted ETag from the revision number and metadata freshness.
+func computeETag(rev *db.Revision, meta *db.ItemMetadata) string {
+	var ts int64
+	if meta != nil {
+		ts = meta.UpdatedAt.UnixNano()
+	} else {
+		ts = rev.CreatedAt.UnixNano()
+	}
+	raw := fmt.Sprintf("%d:%d", rev.RevisionNumber, ts)
+	h := sha256.Sum256([]byte(raw))
+	return `"` + hex.EncodeToString(h[:8]) + `"`
+}
+
+// canSkipRepack returns true if the stored blob already has up-to-date silo/ data.
+func canSkipRepack(rev *db.Revision, meta *db.ItemMetadata) bool {
+	if meta == nil {
+		return true // no metadata row = plain .fcstd
+	}
+	if meta.RevisionHash != nil && rev.FileChecksum != nil &&
+		*meta.RevisionHash == *rev.FileChecksum &&
+		meta.UpdatedAt.Before(rev.CreatedAt) {
+		return true
+	}
+	return false
+}
+
+// derefStr returns the value of a *string pointer, or fallback if nil.
+func derefStr(p *string, fallback string) string {
+	if p != nil {
+		return *p
+	}
+	return fallback
+}