fix: Makefile DB targets follow POSTGRES_USER/DB; warn on POSTGRES_DB coupling

- make psql/backup/extensions derived DB_USER/DB_NAME from POSTGRES_USER/
  POSTGRES_DB via $(or ...), so renaming them in .env no longer breaks targets
- .env.example: note that changing POSTGRES_DB requires updating cron.database_name
  and pg_partman_bgw.dbname in postgresql.conf

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: programmerShinobi

.env.example

@@ -11,6 +11,8 @@ COMPOSE_PROJECT_NAME=local
 # ---- Database credentials --------------------------------------------------
 POSTGRES_USER=local_dev
 POSTGRES_PASSWORD=CHANGE_ME_use_a_strong_random_password
+# NOTE: if you change POSTGRES_DB, also update `cron.database_name` and
+# `pg_partman_bgw.dbname` in config/postgresql.conf to match (they're coupled).
 POSTGRES_DB=local_db
 
 # ---- Networking ------------------------------------------------------------

Makefile

@@ -11,8 +11,10 @@ endif
 
 DC          := docker compose
 SERVICE     := postgres
-DB_USER     ?= local_dev
-DB_NAME     ?= local_db
+# Derive from .env (POSTGRES_USER/POSTGRES_DB) so renaming them never breaks
+# `make psql` / `backup` / `extensions`. Falls back to the defaults if unset.
+DB_USER     ?= $(or $(POSTGRES_USER),local_dev)
+DB_NAME     ?= $(or $(POSTGRES_DB),local_db)
 TS          := $(shell date +%Y%m%d_%H%M%S)
 
 .DEFAULT_GOAL := help