Raise tail pad to 3s so short transmissions are not clipped

The recording window is anchored to OP25 control-channel timestamps, but
the buffered audio lags those by roughly 1.5s. Trim logs across seven
calls measured the offset at 0.84-1.62s, consistently present.

At a 1.0s pad a short call closed its window before the voice arrived:
a 0.97s control-channel call closed at T+1.97 while voice started around
T+1.5, capturing ~0.4s of speech and cutting mid-word. Confirmed by a
0.57s file whose final 0.10s measured -12.2dB against its own -18.2dB
average - clipped speech, not a tail - and by two short calls that
logged no trim at all because no trailing silence remained.

Being generous is free here: trim_silence already strips trailing
silence back to the guard margin before upload, so long calls are
unaffected while short ones gain the window they need. Over-capture
costs nothing; under-capture loses words permanently.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

.env.example

@@ -38,9 +38,13 @@ CALL_IDLE_TIMEOUT=3
 
 # Seconds of audio kept after the last transmission ends. This is the only
 # headroom protecting the final word of a transmission — usually the disposition
-# or the address. Measured at 0.5s it left ~0.3s of real margin and one recording
-# ended mid-word, hence 1.0.
-CALL_TAIL_PAD_SECONDS=1.0
+# or the address. Raised 1.0 -> 3.0 after field measurement showed the
+# grant-to-speech offset is ~0.84-1.62s (typically ~1.5s): at 1.0s pad, short
+# calls had their recording window close before the voice even started,
+# clipping speech mid-word. Safe to be generous — trim_silence already strips
+# the extra back off long calls before upload, so only short transmissions
+# actually benefit from the larger window.
+CALL_TAIL_PAD_SECONDS=3.0
 
 # Strip leading/trailing dead air before upload. ~63% of an untrimmed recording
 # is silence, which costs Whisper spend and makes it hallucinate text that was