chore: Add initial PostgreSQL configuration files and Docker setup

postgres/.env.example

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+POSTGRES_USER=postgres
+POSTGRES_PASSWORD=postgres
+POSTGRES_DB=postgres
+POSTGRES_PORT=5432

postgres/Dockerfile

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+FROM postgres:18.3-alpine

postgres/compose.yml

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+services:
+  postgres:
+    build:
+      context: .
+      dockerfile: Dockerfile
+    restart: unless-stopped
+    environment:
+      POSTGRES_USER: ${POSTGRES_USER:-postgres}
+      POSTGRES_PASSWORD: ${POSTGRES_PASSWORD:-postgres}
+      POSTGRES_DB: ${POSTGRES_DB:-postgres}
+    ports:
+      - "${POSTGRES_PORT:-5432}:5432"
+    volumes:
+      - postgres_data:/var/lib/postgresql/data
+    healthcheck:
+      test: ["CMD-SHELL", "pg_isready -U ${POSTGRES_USER:-postgres} -d ${POSTGRES_DB:-postgres}"]
+      interval: 10s
+      timeout: 5s
+      retries: 5
+
+volumes:
+  postgres_data: