Phase 9: two-phase garbage collection engine

GC engine (internal/gc/): Collector.Run() implements the two-phase
algorithm — Phase 1 finds unreferenced blobs and deletes DB rows in
a single transaction, Phase 2 deletes blob files from storage.
Registry-wide mutex blocks concurrent GC runs. Collector.Reconcile()
scans filesystem for orphaned files with no DB row (crash recovery).

Wired into admin_gc.go: POST /v1/gc now launches the real collector
in a goroutine with gc_started/gc_completed audit events.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

internal/gc/errors.go

@@ -0,0 +1,6 @@
+package gc
+
+import "errors"
+
+// ErrGCRunning indicates that a GC run is already in progress.
+var ErrGCRunning = errors.New("gc: already running")