Add self-hosted Postgres docker-compose

- docker-compose.yml: Postgres 16 Alpine with named volume, healthcheck
- .env.example: POSTGRES_* and DATABASE_URL template
- .gitignore: exclude db_data/ volume

Starting fresh, no data migration. App still wired to Supabase;
DB is ready for migrations to be applied.

.env.example

@@ -0,0 +1,15 @@
+# --- Self-hosted Postgres (Docker) ---
+POSTGRES_USER=routecommerce
+POSTGRES_PASSWORD=change-me-strong-password
+POSTGRES_DB=route_commerce
+# Used by the app and push-migrations.js to talk to the local DB
+DATABASE_URL=postgresql://routecommerce:change-me-strong-password@127.0.0.1:5432/route_commerce?schema=public
+
+# --- Supabase (legacy, can be removed once app is rewired) ---
+NEXT_PUBLIC_SUPABASE_URL=
+NEXT_PUBLIC_SUPABASE_ANON_KEY=
+SUPABASE_SERVICE_ROLE_KEY=
+
+# --- App secrets ---
+MINIMAX_API_KEY=
+MINIMAX_BASE_URL=