fix: replace process log SSE with incremental polling

[milkway]

Closes #1039.

Root cause

The process Logs page automatically held an infinite EventSource response open. In the production path, the load balancer talks HTTP/1.1 to nginx; a stale/reused backend connection could retain that response and head-of-line block a later admin navigation, which explains the UI hanging after several tab changes.

What changed

• replace the automatic process-log SSE feed with finite cursor-based polling
• capture the initial tail and cursor under one lock so no line can fall between the server render and first poll
• stop polling while paused or hidden, prevent overlapping requests, and retry with bounded backoff
• return 204 No Content from the legacy stream route so pre-deploy EventSource clients stop reconnecting during rollout
• preserve admin-only access and add regression coverage for cursor handoff, response shape, caching, legacy shutdown, and RBAC

Container-log live follow remains explicitly user-activated, and image-pull event streams remain finite.

Validation

• DOCKER_REGISTRY_PASSWORD=test cargo test -p ruscker-admin
• DOCKER_REGISTRY_PASSWORD=test cargo clippy --all-targets -- -D warnings
• ./scripts/i18n-check.sh
• git diff --check