fix(node,hub): HEVC transcode fallback, live-add progress, per-group TMDB toggle

Three bugs found live testing the Videos app against a real HEVC/EAC3 show,
plus a design change requested afterward:

- Streaming always did "-c:v copy", which faithfully reports a source's real
  hev1 codec string but is unplayable in a browser with no HEVC decoder
  (most Chrome/Linux builds). The node now transcodes to H264 whenever the
  probed codec is browser-incompatible (media_probe.py's new
  BROWSER_INCOMPATIBLE_VIDEO_CODECS), with a `transcode_incompatible_video`
  node.toml opt-out for operators who know their viewers already decode it.

- Dropping a whole season into an already-watched folder gave no scanning
  indicator and no progress bar: IndexProgress was only ever updated by the
  two bulk scan paths, never by the real-time per-file watchdog path
  (_schedule_update/_debounce/_update_entry). That path now accounts a
  "burst" the same way, without double-counting a file rewritten mid-debounce.

- A stray literal "0" rendered in the video detail modal when there was no
  TMDB match (`meta.confidence` is 0, and `0 && x` renders "0" in JSX/htm,
  not nothing) — `confident` is now a real boolean.

- Whether TMDB is used at all moves from a node-wide setting to per-group
  (OP_TMDB_ENABLED/tmdb_enabled/tmdb_enabled_ack, scoped like OP_VIDEO_ROOT):
  an operator running a real media-library group alongside test/demo groups
  on one node wants outbound TMDB traffic for the one that needs it, not all
  of them. The custom API token and query language stay node-wide, one
  shared credential/cache (tmdb_config/OP_TMDB_CONFIG, unchanged reasoning).
  MNP_VERSION 0.6 -> 0.7, additive.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LAmyXtc6dAADsH23ydXQpY

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