Introduce a pluggable metadata persistence layer: a MetaStore of typed
repositories (AccountRepo, ProjectRepo, OrgRepo, ShareRepo, DeviceRepo).
The five registries keep their in-memory maps + logic and now persist each
change as one record through a repo, instead of rewriting a whole JSON file
inline. The `file` backend (db_file.go) reproduces the exact on-disk JSON
formats, so a running hub loads unchanged. Open*(path) constructors stay as
thin wrappers over the file backend.
Zero behavior change: full webapp suite green; the example hub boots and
existing auth.json/orgs.json/projects.json load and sign-in works.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01R7Q9ZKSZRTdvrSJkYLUmYs
