Update Docs and Compose to Use Harbor Registry

[bupd]

Summary

• Add Docker Images section to README with registry info
• Update quickstart with pre-built image option (recommended)
• Document image tag differences (latest vs versioned)
• Update docker-compose.yml to pull from Harbor

Changes

README

• Added "Docker Images" section with pull commands
• Harbor as primary: registry.goharbor.io/bupd/shitpost
• GHCR as alternative: ghcr.io/bupd/shitpost
• Tag table explaining latest vs versioned tags
• Split quickstart into Option 1 (pre-built) and Option 2 (build from source)

docker-compose.yml

• Changed image from bupd/shitpost-engine:latest to registry.goharbor.io/bupd/shitpost:latest
• Removed build directive (users pull pre-built image by default)