Server: harden multi-vault scoped reads and sync APIs

[TristanEDU]

Goal

Harden the hosted MCP server for multi-vault private alpha use.

Scope

• Keep tenant/vault/installation scoping on all persisted documents, manifests, OAuth/session state, and write proposals.
• Preserve default-vault behavior for single-vault users.
• Prevent cross-vault leakage in search, list, fetch, status, and debug flows.
• Keep /admin/sync as a compatibility alias during migration.
• Add migration-safe Postgres schema handling as V2 tables evolve.

Done when

• Multi-vault tests cover isolation and default behavior.
• Remote smoke can target a specific vault id.
• Server APIs are documented in the platform repo and docs repo.