fix(compose): mount postgres volume at /var/lib/postgresql for pg 18

Postgres 18+ stores data in a version-specific subdirectory and aborts
on startup when a volume is mounted at the legacy /var/lib/postgresql/data
path. The pg16->pg18 bump left the old mount in place, so the container
exited(1) before the healthcheck could run. Mount at the parent dir as
the image recommends.

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

Author: trevex

deploy/compose/dev.yml

@@ -11,7 +11,9 @@ services:
     ports:
       - "5432:5432"
     volumes:
-      - knot-dev-postgres:/var/lib/postgresql/data
+      # pg 18+ stores data in a version-specific subdir; mount at the parent,
+      # not /var/lib/postgresql/data (the image aborts on the legacy path).
+      - knot-dev-postgres:/var/lib/postgresql
     healthcheck:
       test: ["CMD-SHELL", "pg_isready -U knot -d knot"]
       interval: 2s