fix(server): publish newly added spaces

[teloverge]

What changed

• notify the existing poller after a newly added space is persisted
• replace the add-then-manual-refresh test with an add-only publication regression test
• document the design, implementation plan, and user-visible Unreleased change

Why

Adding a GitHub repository saved it to the space store, but the add endpoint did not wake synchronization. The sidebar renders authoritative models from the control WebSocket, so the new project remained invisible until restart, removal, manual refresh, or the next scheduled poll.

Impact

A successfully added repository now appears through the next authoritative model publication without another space action. Validation, persistence-error behavior, response shape, routing, and stale/provider-error semantics are unchanged.

Validation

• cargo.exe fmt --all -- --check
• cargo.exe test -p stellr-server --test api_test --locked -- --test-threads=1 (22 passed)
• cargo.exe clippy --workspace --all-targets --locked -- -D warnings
• cargo.exe test --workspace --locked -- --test-threads=1
• independent code review: no Critical, Important, or Minor findings