fix(hub): stop Search-view video posters flickering to spinners

The cross-group "Search files" view caps its WebRTC connection pool at
MAX_POOL_SIZE but pre-connected every indexed group, so any account with
more groups than the cap thrashed the pool. An evicted transport was
never removed from SearchPage's own groupConns map, so every tile of that
group kept a `_tRef` pointing at a closed transport; MediaThumb and
useMediaMeta bail on a disconnected transport with no retry, so posters
rendered for a moment then fell back to a loading spinner for good. Every
connect also fired the module-wide bumpMediaMetaGeneration(), clearing
every mounted tile's metadata across all groups and flickering the whole
grid through the warm-up walk. A one-group account (grenet) never hit it;
a many-group account (cbesson) always did.

- ConnectionPool takes an onEvict callback; SearchPage prunes groupConns
  and rebuilds the entry lists when a connection is evicted or closed.
- MAX_POOL_SIZE 3 -> 12; the pre-connect walk is capped to it. Groups
  past the cap connect lazily when a tile scrolls into view (onNeedConn).
- connectGroup() no longer fires the module-wide meta/thumb generation
  bumps on a normal connect (kept for operator TMDB override/rematch).
  Per-group gen counter + concurrent-caller guard: a group's tiles
  refetch once per (re)connect, not once per mounting tile.
- MediaThumb/useMediaMeta take an optional reloadKey; each group's
  `_connGen` is threaded through so a tile refetches when its group
  reconnects instead of staying stuck on a spinner.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01W4Yj8EuXkjjYxbeS5kPd3U

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