FIX: MessageBus reconnect reliability

[merefield]

• Reworked live status updates to be MessageBus-first, with notification and PM badges seeded from Discourse’s notification_channel_position instead of relying on steady-state HTTP polling.
• Replaced constant healthy-state badge polling with reconnect-triggered one-shot HTTP resync, while keeping HTTP fallback when live counts are still unseeded or MessageBus is unavailable.
• Added per-channel MessageBus cursor tracking for /latest, /new, /unread, /unread/:user_id, /notification/:user_id, and active /topic/:id subscriptions.
• Added a MessageBus watchdog that runs every 30s, detects stale clients after 240s without successful polls, and rebuilds the client cleanly.
• Added reconnect recovery that restores subscriptions from saved channel positions and requests list/topic refreshes when resume state is not trustworthy.
• Introduced a timeout-configured MessageBusHTTPClient wrapper with explicit open, read, and write timeouts.
• Fixed reconnect edge cases so badge seeding keeps retrying until live values are populated, and default -1 cursors are treated as non-resumable rather than trusted.
• Added live topic subscriptions for /topic/:id, so termcourse can react while you are inside a topic instead of only at the list level.
• Added live in-topic handling for new posts, including tail-follow behavior when you are already on the last loaded post.
• Added live in-topic handling for post edits, likes/unlikes, delete/recover-style updates via targeted single-post refreshes.
• Added topic refresh fallback for reload-style and stats-style topic events when incremental updates are not sufficient.
• Fixed topic reconnect behavior so revisiting a topic updates the stored topic cursor and failed topic refreshes stay pending until they succeed.
• Improved topic-view responsiveness by removing constant idle rerenders, rendering only the needed visible post blocks, caching rendered post blocks, and reducing debug I/O churn.
• Tightened temporary debug instrumentation so it is only active when TERMCOURSE_DEBUG=1.
• Clarified auth behavior in the README: realtime MessageBus features require login auth with a session cookie; API-key auth is HTTP-only and does not support live PM/topic updates.
• Added focused regression coverage for reconnect reliability, resumable cursors, topic refresh behavior, healthy-state no-poll behavior, and the custom MessageBus HTTP client wrapper.