feat: PostgreSQL health gates app start (E00-S02-T02)

compose.yaml

@@ -1,10 +1,14 @@
-# EPPP Docker Compose baseline — [E00-S02-T01]
+# EPPP Docker Compose baseline — [E00-S02-T01/T02]
 #
 # `docker compose up -d` starts both the database (PostgreSQL) and the
 # application (@personal-blog/server). Rollback: `docker compose down`.
 #
-# Explicitly out of scope for T01 (land in later E00-S02 tasks):
-#   - PostgreSQL health gate (T02)
+# PostgreSQL health gate (T02): the `db` service carries a `pg_isready`
+# healthcheck and the `app` service depends on it with
+# `condition: service_healthy`, so the application does not start until the
+# database is accepting connections.
+#
+# Explicitly out of scope for T01/T02 (land in later E00-S02 tasks):
 #   - application health endpoint (T03)
 #   - DB volume persistence (T04)
 #
@@ -22,6 +26,15 @@ services:
       POSTGRES_PASSWORD: ${POSTGRES_PASSWORD:-eppp}
     ports:
       - "${POSTGRES_PORT:-5432}:5432"
+    # Health gate for the app service (T02): probe the same credentials the db
+    # service was created with. `$$` defers interpolation to the container, so
+    # POSTGRES_USER/POSTGRES_DB overrides apply to the probe too.
+    healthcheck:
+      test: ["CMD-SHELL", "pg_isready -U $${POSTGRES_USER} -d $${POSTGRES_DB}"]
+      interval: 5s
+      timeout: 5s
+      retries: 5
+      start_period: 5s
 
   app:
     build:
@@ -31,5 +44,8 @@ services:
       DATABASE_URL: postgres://eppp:eppp@db:5432/eppp
     ports:
       - "${APP_PORT:-3000}:3000"
+    # T02: start only once the database reports healthy (service_healthy), so
+    # the application waits for PostgreSQL before starting.
     depends_on:
-      - db
+      db:
+        condition: service_healthy