feat(acars): add message translator and ADS-B datalink integration

Add ACARS label translation, message classification, and field parsers
so decoded messages show human-readable descriptions instead of raw
label codes (H1, DF, _d, 5Z, etc.). Integrate translated ACARS
messages into the ADS-B aircraft detail panel and add a live message
feed to the standalone ACARS mode.

- New utils/acars_translator.py with ~50 label codes, type classifier,
  and parsers for position reports, engine data, weather, and OOOI
- Enrich messages at ingest in routes/acars.py with translation fields
- Backfill translation in /adsb/aircraft/<icao>/messages endpoint
- ADS-B dashboard: DATALINK MESSAGES section in aircraft detail panel
  with auto-refresh, color-coded type badges, and parsed field display
- Standalone ACARS mode: scrollable live message feed (max 30 cards)
- Fix default N. America ACARS frequencies to 131.550/130.025/129.125
- Unit tests covering all translator functions

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.env.example

@@ -1,2 +1,32 @@
-# Uncomment and set to use external storage for ADS-B history
-# PGDATA_PATH=/mnt/external/intercept/pgdata
+# =============================================================================
+# INTERCEPT CONTROLLER (.env)
+# =============================================================================
+# Copy to .env and edit for your setup
+
+# Container timezone (e.g. America/New_York, Europe/London, Australia/Sydney)
+TZ=UTC
+
+# Postgres password (default: intercept)
+INTERCEPT_ADSB_DB_PASSWORD=intercept
+
+# Auto-start ADS-B when dashboard loads
+INTERCEPT_ADSB_AUTO_START=false
+
+# Share observer location across all modules
+INTERCEPT_SHARED_OBSERVER_LOCATION=true
+
+# Observer coordinates (uncomment and set to skip GPS prompt)
+# INTERCEPT_DEFAULT_LAT=40.7128
+# INTERCEPT_DEFAULT_LON=-74.0060
+
+# =============================================================================
+# AGENT SETTINGS (for docker-compose.agent.yml on remote Pis)
+# =============================================================================
+
+# Agent identity
+AGENT_NAME=sdr-agent-1
+AGENT_PORT=8020
+
+# Controller connection (IP of the machine running docker-compose.yml)
+CONTROLLER_URL=http://192.168.1.100:5050
+AGENT_API_KEY=changeme