refactor(compose): make app/db ports configurable via DB_PORT and SERVER_PORT

Replace hardcoded port literals with env-driven variables so a single change
in .env reconfigures the full stack consistently. Defaults are unchanged
(DB 5432, app 8765), so existing setups need no .env changes.

- DB_PORT (default 5432): drives the db host<->container port mapping,
  Postgres PGPORT (so it listens on the chosen port), and the app's
  DATABASE_PORT connection setting. Lets deployers avoid a host port conflict
  (e.g. another local Postgres) by setting DB_PORT once.
- SERVER_PORT (default 8765): drives the app host<->container mapping, the
  SERVER_PORT the app listens on, and the healthcheck target URL.
- Applied to both the base (docker-compose.yml) and the dev override
  (docker-compose.dev.yml, tests service) so dev and prod stay in sync.

docker-compose.dev.yml

@@ -23,7 +23,7 @@ services:
     environment:
       # Database Configuration
       DATABASE_HOST: db
-      DATABASE_PORT: "5432"
+      DATABASE_PORT: ${DB_PORT:-5432}
       DATABASE_USER: postgres
       DATABASE_PASSWORD: ${DBPASS}
       DATABASE_NAME: bookhoard
@@ -31,7 +31,7 @@ services:
 
       # Application Configuration
       JWT_SECRET: ${JWT_SECRET}
-      SERVER_PORT: "8765"
+      SERVER_PORT: ${SERVER_PORT:-8765}
 
       # Test Configuration
       TEST_MODE: "true"