Reusing `_MEDIA_SEND_READ_TIMEOUT` (60 s) as the whole-call deadline turned
httpx's per-phase stall budget into a bandwidth cap: a 20 MB video on a
~2 Mbit/s uplink (~80 s) that succeeds today would fail. `_MEDIA_SEND_DEADLINE`
= 300 s covers the 50 MB Bot API cap at ~2 Mbit/s plus connect and sendVideo
transcoding, and is >2x the summed httpx budgets (pool 8 + connect 10 +
media_write 60 + read 60 = 138 s), so it only fires on a socket that has
stopped raising. Abandon-inside-lock semantics documented at the constant.
85e673b4a5