Phases 5, 6, 8: OCI pull/push paths and admin REST API

Phase 5 (OCI pull): internal/oci/ package with manifest GET/HEAD by
tag/digest, blob GET/HEAD with repo membership check, tag listing with
OCI pagination, catalog listing. Multi-segment repo names via
parseOCIPath() right-split routing. DB query layer in
internal/db/repository.go.

Phase 6 (OCI push): blob uploads (monolithic and chunked) with
uploadManager tracking in-progress BlobWriters, manifest push
implementing full ARCHITECTURE.md §5 flow in a single SQLite
transaction (create repo, upsert manifest, populate manifest_blobs,
atomic tag move). Digest verification on both blob commit and manifest
push-by-digest.

Phase 8 (admin REST): /v1 endpoints for auth (login/logout/health),
repository management (list/detail/delete), policy CRUD with engine
reload, audit log listing with filters, GC trigger/status stubs.
RequireAdmin middleware, platform-standard error format.

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

internal/db/errors.go

@@ -0,0 +1,9 @@
+package db
+
+import "errors"
+
+var (
+	ErrRepoNotFound     = errors.New("db: repository not found")
+	ErrManifestNotFound = errors.New("db: manifest not found")
+	ErrBlobNotFound     = errors.New("db: blob not found")
+)