feat: persist the database volume across restart/recreate (E00-S02-T04)

Mount the named `db-data` volume at PostgreSQL's data directory
(/var/lib/postgresql/data) on the db service and declare it in the
top-level volumes map, so the database survives `docker compose
restart` and `docker compose down` + `up -d` (recreate). Reset with
`docker compose down -v` per the issue rollback note. Header comments
in compose.yaml and the server Dockerfile updated: T04 is no longer out
of scope; T05/T06 remain.

compose.yaml

@@ -1,4 +1,4 @@
-# EPPP Docker Compose baseline — [E00-S02-T01/T02/T03]
+# EPPP Docker Compose baseline — [E00-S02-T01/T02/T03/T04]
 #
 # `docker compose up -d` starts both the database (PostgreSQL) and the
 # application (@personal-blog/server). Rollback: `docker compose down`.
@@ -12,8 +12,14 @@
 # `{"status":"ok"}`) on port 3000, so the app container stays up and the
 # health endpoint succeeds once the stack is running.
 #
-# Explicitly out of scope for T01/T02/T03 (land in later E00-S02 tasks):
-#   - DB volume persistence (T04)
+# Database volume persistence (T04): the `db` service mounts the named volume
+# `db-data` at PostgreSQL's data directory (`/var/lib/postgresql/data`), so
+# the database survives `docker compose restart` (restart) and
+# `docker compose down` + `docker compose up -d` (recreate, which discards the
+# container filesystem). Reset the data with `docker compose down -v`.
+#
+# Explicitly out of scope for T01..T04 (land in later E00-S02 tasks):
+#   - non-root execution (T05), read-only root filesystem (T06)
 #
 # All values have defaults so `docker compose up -d` works from a clean clone
 # without a .env file (a committed .env.example template lands in E00-S04).
@@ -29,6 +35,11 @@ services:
       POSTGRES_PASSWORD: ${POSTGRES_PASSWORD:-eppp}
     ports:
       - "${POSTGRES_PORT:-5432}:5432"
+    # T04: persist the database in the named `db-data` volume (PostgreSQL data
+    # directory), so data survives `docker compose restart` and `docker
+    # compose down` + `up -d` (recreate). `docker compose down -v` resets it.
+    volumes:
+      - db-data:/var/lib/postgresql/data
     # Health gate for the app service (T02): probe the same credentials the db
     # service was created with. `$$` defers interpolation to the container, so
     # POSTGRES_USER/POSTGRES_DB overrides apply to the probe too.
@@ -52,3 +63,9 @@ services:
     depends_on:
       db:
         condition: service_healthy
+
+# Named volumes shared across `docker compose` lifecycles. `db-data` (T04)
+# holds the PostgreSQL data directory and is preserved across restart and
+# recreate; `docker compose down -v` removes it to reset the database.
+volumes:
+  db-data: