feat(infra): add docker-compose for local MariaDB with Prisma schema init

Applies prisma/migrations/*/migration.sql on first container boot via a
sourced docker-entrypoint-initdb.d script, so a fresh container matches a
`prisma migrate deploy` run without needing shadow-database privileges.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

docker-compose.yml

@@ -0,0 +1,28 @@
+services:
+  mariadb:
+    image: mariadb:lts
+    container_name: convert_mariadb
+    restart: unless-stopped
+    # Credentials below match .env.local (DB_HOST=127.0.0.1, DB_USER, DB_PASSWORD, DB_NAME).
+    # Update both files together if you change any of these.
+    environment:
+      MARIADB_ROOT_PASSWORD: root_change_me
+      MARIADB_DATABASE: file_converter
+      MARIADB_USER: convert_user
+      MARIADB_PASSWORD: change_me
+    ports:
+      - "3306:3306"
+    volumes:
+      - mariadb_data:/var/lib/mysql
+      # Raw SQL from prisma/migrations, applied on first boot by the init script below.
+      - ./prisma/migrations:/prisma-migrations:ro
+      - ./docker/mariadb-initdb:/docker-entrypoint-initdb.d:ro
+    healthcheck:
+      test: ["CMD", "healthcheck.sh", "--connect", "--innodb_initialized"]
+      start_period: 10s
+      interval: 5s
+      timeout: 5s
+      retries: 10
+
+volumes:
+  mariadb_data: