chore(dev): align dev Postgres to pg18 matching production

Production runs Postgres 18; the dev compose file pinned pg16. A pg_dump
from prod (v18) cannot be restored by a v16 pg_restore — the dump header
is rejected. Matching the major version fixes restores and removes a
latent source of dev/prod behaviour drift.

Also adopts the pg18 image's data-directory convention: the volume now
mounts at /var/lib/postgresql (the image places data in a version
subdirectory), and drops the obsolete compose 'version' key.

docker-compose.dev.yml

@@ -1,8 +1,11 @@
-version: '3.8'
-
+# Local development services for Tia (Postgres + Redis).
+# Postgres major version is pinned to match production.
 services:
   postgres:
-    image: pgvector/pgvector:pg16
+    # pg18 to match production (Hetzner prod runs Postgres 18).
+    # Keeping dev on the same major version avoids dump/restore mismatches
+    # and version-specific behaviour drift.
+    image: pgvector/pgvector:pg18
     container_name: tia-postgres
     environment:
       POSTGRES_DB: tia_dev
@@ -11,7 +14,9 @@ services:
     ports:
       - "5433:5432"
     volumes:
-      - ./data/postgres:/var/lib/postgresql/data
+      # pg18 image convention: mount at /var/lib/postgresql (NOT .../data).
+      # The image places data in a version-specific subdirectory under it.
+      - ./data/postgres:/var/lib/postgresql
       - ./docker/init-db:/docker-entrypoint-initdb.d
     healthcheck:
       test: ["CMD-SHELL", "pg_isready -U tia -d tia_dev"]