platform ops: disk usage, backup/export, and shared init

[sdelcore]

Stacked on #5 (chain: #3 ← #5 ← this). Base this PR's review on top of the versioning/rollback/delete work; merge #3 and #5 first.

Three operational features for the shared platform.

1. Per-site disk usage

GET /api/sites now includes a bytes field per site — the summed size of the site's sites/, db/, uploads/, and versions/ trees (regular files only, race errors ignored). The homepage renders it as a human-readable size (B/KB/MB/GB, one decimal) next to the existing updated-at, and shared list prints it as a middle column.

2. Backup / export

New GET /api/export streams a gzipped tarball of the whole data directory (sites, db, uploads, versions, identity.json if present), skipping transient deploy-* / old-* / *.old-* swap dirs. New shared backup [file] downloads it, defaulting to shared-backup-<yyyymmdd-hhmmss>.tar.gz in the cwd and printing the path and size.

3. shared init

New shared init [dir] scaffolds index.html (loads /shared.js, demonstrates shared.identity()) and .claude/skills/shared-sites/SKILL.md (an agent skill documenting the full client API + deploy flow), both embedded in the binary via internal/web. Refuses to overwrite existing files, skipping with a note.

Why

Operational visibility (how much disk each site uses), a one-command backup for the plain-files data dir, and a fast on-ramp for building new sites with accurate, in-repo API docs for coding agents.

Verification

• go build ./..., go vet ./..., gofmt -l . (clean), and nix build all pass.
• Live server against scratch SHARED_DATA: deployed a site with a known-size file plus a db doc and an upload → /api/sites bytes reflected it and the homepage rendered the size string; shared backup produced a tarball listing the site files, db json, and upload; shared init created both files in a fresh dir, a second run skipped them with a note, and the scaffolded index.html deployed and loaded (200).

File boundaries

Did not touch internal/server/ai.go, the AI parts of internal/web/shared.js, or the README AI/env-AI rows (owned by a parallel branch). server.go change is limited to one route registration.