Make Postgres data path configurable for ADS-B history

Allow users to override the pgdata volume mount via PGDATA_PATH env var,
enabling external storage (e.g. USB) for ADS-B history. Defaults to
./pgdata for backwards compatibility.

Based on PR #88 by JamesIOmete, rebased cleanly onto main.

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

docker-compose.yml

@@ -94,7 +94,8 @@ services:
       - POSTGRES_USER=intercept
       - POSTGRES_PASSWORD=intercept
     volumes:
-      - ./pgdata:/var/lib/postgresql/data
+      # Default local path (override with PGDATA_PATH for external storage)
+      - ${PGDATA_PATH:-./pgdata}:/var/lib/postgresql/data
     restart: unless-stopped
     healthcheck:
       test: ["CMD-SHELL", "pg_isready -U intercept -d intercept_adsb"]