feat: add example datasets and workflows to Docker Compose

[bobbai00]

What changes were proposed in this PR?

This PR adds example datasets and workflows that are automatically loaded into Texera when using Docker Compose, so new users have sample data to explore immediately after startup.

What gets loaded:

• 2 datasets: Iris Species (5KB CSV), TMDb Popular Movies (327KB CSV)
• 2 workflows: ML on Iris Dataset, Data Exploration on Movies Dataset

Key design choices:

• Uses Docker Compose profiles — the loader only runs when explicitly opted in via docker compose --profile examples up, not on every restart
• Uses a stock alpine:latest image with runtime apk add — no custom image build required
• Idempotent — skips datasets/workflows that already exist
• Admin credentials (USER_SYS_ADMIN_USERNAME/USER_SYS_ADMIN_PASSWORD) are defined in .env and shared between the Texera backend and the example loader

Files added:

• bin/single-node/examples/ — datasets (CSV + descriptions), workflow JSONs, and load-examples.sh loader script
• bin/single-node/docker-compose.yml — added example-data-loader service (Part 4)
• bin/single-node/.env — added USER_SYS_ADMIN_USERNAME/USER_SYS_ADMIN_PASSWORD env vars

Any related issues, documentation, discussions?

No

How was this PR tested?

1. Ran docker compose --profile examples up and verified:
   • The loader waits for services to be healthy
   • Datasets and workflows appear in the Texera UI
   • The loader container exits after loading
2. Ran docker compose up (without --profile examples) and verified the loader does NOT start

Was this PR authored or co-authored using generative AI tooling?

Generated-by: Claude Code (Claude Opus 4.6)

[chenlica]

@kunwp1 Can you review it?