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/server/admin_gc.go

@@ -1,10 +1,15 @@
 package server
 
 import (
+	"context"
+	"fmt"
 	"net/http"
 	"sync"
+	"time"
 
 	"github.com/google/uuid"
+
+	"git.wntrmute.dev/kyle/mcr/internal/gc"
 )
 
 // GCLastRun records the result of the last GC run.
@@ -17,9 +22,11 @@ type GCLastRun struct {
 
 // GCState tracks the current state of garbage collection.
 type GCState struct {
-	mu      sync.Mutex
-	Running bool       `json:"running"`
-	LastRun *GCLastRun `json:"last_run,omitempty"`
+	mu        sync.Mutex
+	Running   bool       `json:"running"`
+	LastRun   *GCLastRun `json:"last_run,omitempty"`
+	Collector *gc.Collector
+	AuditFn   AuditFunc
 }
 
 type gcStatusResponse struct {
@@ -43,10 +50,51 @@ func AdminTriggerGCHandler(state *GCState) http.HandlerFunc {
 		state.Running = true
 		state.mu.Unlock()
 
-		// GC engine is Phase 9 -- for now, just mark as running and return.
-		// The actual GC goroutine will be wired up in Phase 9.
 		gcID := uuid.New().String()
 
+		// Run GC asynchronously.
+		go func() {
+			startedAt := time.Now().UTC().Format(time.RFC3339)
+
+			if state.AuditFn != nil {
+				state.AuditFn("gc_started", "", "", "", "", map[string]string{
+					"gc_id": gcID,
+				})
+			}
+
+			var result *gc.Result
+			var gcErr error
+			if state.Collector != nil {
+				result, gcErr = state.Collector.Run(context.Background())
+			}
+
+			completedAt := time.Now().UTC().Format(time.RFC3339)
+
+			state.mu.Lock()
+			state.Running = false
+			lastRun := &GCLastRun{
+				StartedAt:   startedAt,
+				CompletedAt: completedAt,
+			}
+			if result != nil {
+				lastRun.BlobsRemoved = result.BlobsRemoved
+				lastRun.BytesFreed = result.BytesFreed
+			}
+			state.LastRun = lastRun
+			state.mu.Unlock()
+
+			if state.AuditFn != nil && gcErr == nil {
+				details := map[string]string{
+					"gc_id": gcID,
+				}
+				if result != nil {
+					details["blobs_removed"] = fmt.Sprintf("%d", result.BlobsRemoved)
+					details["bytes_freed"] = fmt.Sprintf("%d", result.BytesFreed)
+				}
+				state.AuditFn("gc_completed", "", "", "", "", details)
+			}
+		}()
+
 		writeJSON(w, http.StatusAccepted, gcTriggerResponse{ID: gcID})
 	}
 }