Add encrypted contacts across server, Rust, and mobile

[ua741]

Summary

• add the contacts backend on Museum, including root-key reuse, contact CRUD/diff, profile-picture upload/download, eligibility checks, and attachment replication/deletion lifecycle
• add the shared Rust contacts client plus Flutter Rust Bridge surface
• add the shared ente_contacts Flutter package with local DB, sync, wrapped-root-key persistence, and profile-picture caching

Testing

• ENV=test go test ./pkg/controller/contact ./pkg/repo/datacleanup ./pkg/controller/user ./cmd/museum
• cargo test --manifest-path rust/contacts/Cargo.toml
• flutter test && flutter analyze in mobile/packages/contacts

Known blockers

• wrapped root contact key caching is still type-scoped and not cleared on logout, while Rust opens from cache before confirming account identity
• rust/core currently disables TLS cert validation for any non-default base URL, not just local insecure test endpoints

[socket-security]

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