feat(kc): commit extraction pipeline + metadata API (Phase 1)

Implements issue #141 — .kc server-side metadata integration Phase 1.

When a .kc file is uploaded, the server extracts silo/manifest.json and
silo/metadata.json from the ZIP archive and indexes them into the
item_metadata table. Plain .fcstd files continue to work unchanged.
Extraction is best-effort: failures are logged but do not block the upload.

New packages:
- internal/kc: ZIP extraction library (Extract, Manifest, Metadata types)
- internal/db: ItemMetadataRepository (Get, Upsert, UpdateFields,
  UpdateLifecycle, SetTags)

New API endpoints under /api/items/{partNumber}:
- GET    /metadata           — read indexed metadata (viewer)
- PUT    /metadata           — merge fields into JSONB (editor)
- PATCH  /metadata/lifecycle — transition lifecycle state (editor)
- PATCH  /metadata/tags      — add/remove tags (editor)

SSE events: metadata.updated, metadata.lifecycle, metadata.tags

Lifecycle transitions (Phase 1): draft→review→released→obsolete,
review→draft (reject).

Closes #141

internal/api/handlers.go

@@ -53,6 +53,7 @@ type Server struct {
 	modules       *modules.Registry
 	cfg           *config.Config
 	settings      *db.SettingsRepository
+	metadata      *db.ItemMetadataRepository
 }
 
 // NewServer creates a new API server.
@@ -81,6 +82,7 @@ func NewServer(
 	jobs := db.NewJobRepository(database)
 	settings := db.NewSettingsRepository(database)
 	locations := db.NewLocationRepository(database)
+	metadata := db.NewItemMetadataRepository(database)
 	seqStore := &dbSequenceStore{db: database, schemas: schemas}
 	partgen := partnum.NewGenerator(schemas, seqStore)
 
@@ -109,6 +111,7 @@ func NewServer(
 		modules:       registry,
 		cfg:           cfg,
 		settings:      settings,
+		metadata:      metadata,
 	}
 }
 
@@ -1652,6 +1655,9 @@ func (s *Server) HandleUploadFile(w http.ResponseWriter, r *http.Request) {
 		Int64("size", result.Size).
 		Msg("file uploaded")
 
+	// .kc metadata extraction (best-effort)
+	s.extractKCMetadata(ctx, item, fileKey, rev)
+
 	writeJSON(w, http.StatusCreated, revisionToResponse(rev))
 }