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>

.gitignore

@@ -54,3 +54,8 @@ intercept_agent_*.cfg
 # Temporary files
 /tmp/
 *.tmp
+
+# Env files
+.env
+.env.*
+!.env.example