Backsies: multisite support (per-subsite backup and restore, network backup)

Imported from GitHub issue El-SaMa/oma#175 by @El-SaMa.

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Tasks

• Per-subsite backup: tables wp_N_*, the subsite's rows in shared tables (wp_users and wp_usermeta for its users, wp_blogs, wp_site/wp_sitemeta where relevant, domain mapping rows), uploads/sites/N.
• Network backup: full DB + code + uploads, nightly, off-box (restic).
• Single-subsite restore without touching other subsites (to the same blog_id, or to a new one for "restore as copy").
• Schedules and retention per plan; shown in the "My site" page and MCP tools.
• Connector snapshots (W15) are short-term; Backsies is the long-term layer.

Acceptance criteria

• On staging: break one subsite, restore it, others untouched (verified by checksums).

Depends on

El-SaMa/lamoi-waas#2.

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Plan key W21. Epic: El-SaMa/lamoi-waas#25.