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>

drb-edge-node/app/config.py

@@ -47,10 +47,29 @@ class Settings(BaseSettings):
     # Audio kept after the observed end of the last transmission. The srcaddr
     # 1→0 edge can be up to one poll (0.5 s) late and the encoder adds its own
     # latency, so this is the only headroom protecting the last word of a
-    # transmission — which is usually the disposition or the address. Field
-    # measurement at 0.5 s left only 0.29–0.37 s of real trailing margin and one
-    # recording ended mid-word, hence 1.0 s.
-    call_tail_pad_seconds: float = 1.0
+    # transmission — which is usually the disposition or the address.
+    #
+    # Raised 1.0 -> 3.0 after field measurement showed the recording WINDOW
+    # (anchored to OP25 control-channel timestamps) closing well before the
+    # actual voice audio arrives: grant->speech offset measured 0.84-1.62s
+    # across 7 calls (~1.5s typical). At the old 1.0s pad, a short
+    # transmission (e.g. a 0.97s control-channel call) had its window close
+    # at T+1.97 while voice didn't start until ~T+1.5 — leaving ~0.4s of
+    # captured speech, clipped mid-word. Confirmed by a 0.57s output file
+    # whose final 0.10s measured -12.2dB, louder than its own -18.2dB
+    # average (i.e. clipped speech, not trailing silence), and by two short
+    # calls that produced no "Trimmed" log line at all because there was no
+    # trailing silence left to trim.
+    #
+    # Safe to be generous here: trim_silence already strips trailing silence
+    # back to trim_silence_guard_seconds before upload, so a larger pad costs
+    # long calls nothing (the extra is trimmed away) while giving short
+    # transmissions enough window to actually capture the voice. Over-capture
+    # is free; under-capture loses words permanently. Do not tune this back
+    # down without new field data showing the grant->speech offset has
+    # shrunk — see DEFERRED.md for the call_idle_timeout coupling this value
+    # now sits at.
+    call_tail_pad_seconds: float = 3.0
 
     # Strip leading/trailing dead air before upload. ~63% of a typical recording
     # is silence (the grant→speech delay plus the tail pad), which inflates

drb-edge-node/app/internal/metadata_watcher.py

@@ -58,8 +58,13 @@ def _tail_pad() -> float:
     edge (up to one poll late) plus encoder latency never clips the tail.
 
     Read live from settings (env CALL_TAIL_PAD_SECONDS) rather than frozen into a
-    module constant, so it is tunable per node. See the setting for why the
-    default moved 0.5 → 1.0.
+    module constant, so it is tunable per node. See the setting in config.py for
+    why the default moved 1.0 → 3.0 (short calls' recording window was closing
+    before the ~1.5s grant→speech offset let voice audio even start).
+
+    Only the idle-timeout close path below uses this pad — the tgid_change and
+    tgid_change_unlogged paths close at an exact, already-known boundary (the
+    new grant's timestamp, or the same poll tick) and intentionally add none.
     """
     return settings.call_tail_pad_seconds