scrypted: don't reset idle timer on each painted frame + finer timing

Two fixes plus a README refresh:

1. scrypted: pushStreamFrame previously reset the per-stream idle timer
   on every successful /frame response. That made the timer anchored to
   "frames are flowing" rather than to "the camera event that triggered
   the stream", so a continuously-streaming source would never let the
   stream time out. Removed the reset. The startStream → stopStream(false)
   cancel-and-replace path on repeated events still keeps the stream
   alive while the event keeps firing; idle (no new events) now actually
   ends the stream at idle_timeout_ms.

2. firmware: break the previous coarse recv/dec/paint timing into
      lock  : try_lock returned
      ttfb  : first httpd_req_recv chunk landed
      body  : remaining bytes received
      dec   : hardware JPEG decode
      paint : esp_lcd_panel_draw_bitmap returned
      post  : state-counter bookkeeping + unlock
   Logged every 10 frames at INFO. Splits the previously-fat recv bucket
   into TCP/HTTP handshake overhead (ttfb) vs wire-time (body), and
   surfaces any tail bookkeeping cost.

3. README: replace the stale "5 fps ceiling caused by CPU RGB conversion"
   guess with the actual measured per-phase budget and re-rank the
   backlog accordingly. Double-buffering the panel (paint 24 ms → ~2 ms)
   is now the highest-value next move; the previously-listed DMA-2D
   rewrite is moot because the CPU loop is already gone.

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