fix(docs): generate URL-safe Postgres passwords

openssl rand -base64 24 can emit URI-reserved characters (/, +, =). Since POSTGRES_PASSWORD is interpolated unescaped into DATABASE_URL, such a password breaks the connection URL and blocks boot. Switch every documented example to openssl rand -hex 32 (URL-safe, 256-bit).

.env.example

@@ -25,7 +25,7 @@
 DATABASE_URL="postgresql://healthlog:CHANGE-ME@db:5432/healthlog?schema=public"
 
 # Postgres password — used both by the db service AND by DATABASE_URL above.
-# Pick a strong random password, e.g. `openssl rand -base64 24`.
+# Pick a strong random password, e.g. `openssl rand -hex 32`.
 POSTGRES_PASSWORD="CHANGE-ME"