Three root-cause fixes for VLC showing a black screen when opening a
stream from a different network or via Tailscale:
1. PrioritizeTail: when VLC opens an MKV/MP4 stream it immediately seeks
to the end of the file to read the container index (seekhead/moov
atom). For active torrents those end-pieces aren't downloaded yet, so
the reader blocks indefinitely. PrioritizeTail() opens a background
reader positioned at the last 5 MB, keeping those pieces at high
priority until ctx is cancelled or they finish downloading.
2. /health endpoint: GET /health returns a lightweight JSON response
{"status":"ok","streaming":bool,...} so connectivity can be tested
with a simple curl from any device before involving VLC.
3. Per-request logging: every incoming /stream request now logs the
client IP and Range header, making it trivial to confirm whether
remote/Tailscale clients are reaching the server at all.
