fix: align app/db image bases with documented stack (E00-S02-T01)

Switch the app image from node:24-alpine to node:24.19.0-bookworm-slim in
both build and runtime stages (Technology-Stack 5.4: glibc Debian base
required because argon2 is a native dependency; musl/Alpine causes
native-module build surprises), and the db image from postgres:16-alpine
to postgres:18-bookworm (Technology-Stack 5.2/5.4/6.2 + golden tuple 7
pin PostgreSQL 18.6; 18-bookworm is the 18.x line on Debian bookworm).

Update tests/compose-config.test.mjs so the committed assertions lock in
the corrected image bases (db image, Dockerfile build/runtime stages,
parser probe).

compose.yaml

@@ -13,7 +13,9 @@
 
 services:
   db:
-    image: postgres:16-alpine
+    # PostgreSQL 18 on Debian bookworm — the documented runtime target
+    # (Technology-Stack §5.2/§5.4/§6.2, golden tuple §7; no Alpine drift).
+    image: postgres:18-bookworm
     environment:
       POSTGRES_DB: ${POSTGRES_DB:-eppp}
       POSTGRES_USER: ${POSTGRES_USER:-eppp}