Add structured error logging to OCI handlers

Every 500 response in the OCI package silently discarded the actual
error, making production debugging impossible. Add slog.Error before
each 500 response with the error and relevant context (repo, digest,
tag, uuid). Add slog.Info for state-mutating successes (manifest push,
blob upload complete, deletions).

Logger is injected into the OCI Handler via constructor, falling back
to slog.Default() if nil.

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

internal/oci/handler.go

@@ -2,6 +2,7 @@ package oci
 
 import (
 	"io"
+	"log/slog"
 	"net/http"
 	"strings"
 
@@ -61,16 +62,21 @@ type Handler struct {
 	policy  PolicyEval
 	auditFn AuditFunc
 	uploads *uploadManager
+	log     *slog.Logger
 }
 
-// NewHandler creates a new OCI handler.
-func NewHandler(querier DBQuerier, blobs BlobStore, pol PolicyEval, auditFn AuditFunc) *Handler {
+// NewHandler creates a new OCI handler. If logger is nil, slog.Default() is used.
+func NewHandler(querier DBQuerier, blobs BlobStore, pol PolicyEval, auditFn AuditFunc, logger *slog.Logger) *Handler {
+	if logger == nil {
+		logger = slog.Default()
+	}
 	return &Handler{
 		db:      querier,
 		blobs:   blobs,
 		policy:  pol,
 		auditFn: auditFn,
 		uploads: newUploadManager(),
+		log:     logger.With("component", "oci"),
 	}
 }