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>

.dockerignore

@@ -35,6 +35,7 @@ htmlcov/
 
 # Local Postgres data
 pgdata/
+pgdata.bak/
 
 # Captured files (don't include in image)
 *.cap