Fix tail truncation, decouple call length from buffer, trim silence

Measured six recordings off a live P25 node and found two independent
defects causing lost audio at the end of calls:

- stop_recording() sliced the ring buffer immediately, so if the MP3
  muxer had not yet delivered the tail the file was silently short.
  Now waits (bounded, 2s) until buffered audio covers the end epoch.
- TGID-change closes used the new grant's epoch as the end with no pad
  at all, guaranteeing truncation on every split. Tail pad is now a
  setting, default raised 0.5s -> 1.0s.

The ring buffer also capped maximum call length: a call longer than the
buffer had its front silently clamped. The ring now serves the pre-roll
only, with a per-call accumulator for the rest, bounded at 4.8MB.
Clamping is loudly warned rather than silent.

Uploads averaged 63% silence, which inflates STT cost and is a known
Whisper hallucination trigger. Leading/trailing silence is now trimmed
conservatively (-40dB, 0.25s guard, internal pauses untouched).
started_at/ended_at still describe the call; new audio_* fields carry
the trimmed audio bounds so playback can map back to wall clock.
All-silence recordings are skipped and logged instead of uploaded.

Also: log measured control-channel idle on idle-timeout closes so
CALL_IDLE_TIMEOUT can be tuned from data, and quiet the httpx logger
which emitted ~170k lines/day of poll noise.

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

drb-edge-node/app/config.py

@@ -37,8 +37,31 @@ class Settings(BaseSettings):
     # Call segmentation — seconds with no active transmission before the current
     # recording is closed out. Consecutive grants on the SAME talkgroup inside this
     # window are kept in one recording so back-and-forth traffic stays together.
+    #
+    # Do NOT tune this against measured *audio* silence: audio gaps also contain
+    # the ~1.9 s P25 grant→speech delay, so they are always longer than the
+    # control-channel idle this timer measures. metadata_watcher logs the real
+    # measured idle on every idle-timeout close — tune from that.
     call_idle_timeout: float = 3.0
 
+    # 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
+
+    # Strip leading/trailing dead air before upload. ~63% of a typical recording
+    # is silence (the grant→speech delay plus the tail pad), which inflates
+    # Whisper cost and is a well-documented trigger for hallucinated transcript
+    # text. Trimming is conservative — see internal/audio_trim.py.
+    trim_silence: bool = True
+    # Anything quieter than this counts as silence for detection purposes.
+    trim_silence_threshold_db: float = -40.0
+    # Guard margin kept around detected speech so no syllable is clipped.
+    trim_silence_guard_seconds: float = 0.25
+
     # OP25 container
     op25_api_url: str = "http://localhost:8001"
     op25_terminal_url: str = "http://localhost:8081"