feat(03-02): add Docker Compose postgres profiles and build-tagged test helper

- compose.yml: add postgres service with profiles, healthcheck, pg_isready
- compose.yml: add DATABASE_URL env var and conditional depends_on (required: false)
- compose.yml: add postgres-data volume; default deploy remains SQLite-only
- compose.dev.yml: add postgres service with port 5432 exposed for local dev
- compose.dev.yml: add DATABASE_URL env var and conditional depends_on
- pkg/diunwebhook/postgres_test.go: build-tagged NewTestPostgresServer helper

compose.yml

@@ -1,3 +1,4 @@
+# Minimum Docker Compose v2.20 required for depends_on.required
 services:
   app:
     image: gitea.jeanlucmakiola.de/makiolaj/diundashboard:latest
@@ -7,9 +8,33 @@ services:
       - WEBHOOK_SECRET=${WEBHOOK_SECRET:-}
       - PORT=${PORT:-8080}
       - DB_PATH=/data/diun.db
+      - DATABASE_URL=${DATABASE_URL:-}
     volumes:
       - diun-data:/data
     restart: unless-stopped
+    depends_on:
+      postgres:
+        condition: service_healthy
+        required: false
+
+  postgres:
+    image: postgres:17-alpine
+    profiles:
+      - postgres
+    environment:
+      POSTGRES_USER: ${POSTGRES_USER:-diun}
+      POSTGRES_PASSWORD: ${POSTGRES_PASSWORD:-diun}
+      POSTGRES_DB: ${POSTGRES_DB:-diundashboard}
+    volumes:
+      - postgres-data:/var/lib/postgresql/data
+    healthcheck:
+      test: ["CMD-SHELL", "pg_isready -U ${POSTGRES_USER:-diun}"]
+      interval: 5s
+      timeout: 5s
+      retries: 5
+      start_period: 10s
+    restart: unless-stopped
 
 volumes:
   diun-data:
+  postgres-data: