Support Fly.io as a deploy target for containers

[dannywillems]

Problem

Currently catapulta only supports deploying containers to a VPS
via docker save/rsync/load. Lightweight services (webhook relays,
small APIs) could run for free on Fly.io instead of consuming
resources on a paid VPS.

Proposal

Add a Fly.io deploy target using the flyctl CLI. The workflow
would be:

1. Build the Docker image locally
2. Deploy to Fly.io via flyctl deploy

// catapulta.toml or equivalent
[[apps]]
name = "notifier"
provider = "fly"
app = "my-notifier"
region = "cdg"  // Paris
memory = 256

The deployer would:

1. Build the image (existing build_image logic)
2. Call flyctl deploy --local-only --image {tag} or use
   flyctl deploy with a Dockerfile

Fly.io provides:

• Free tier: 3 shared VMs, 256 MB RAM each
• Always-on (no cold starts)
• Built-in HTTPS and custom domains
• Volumes for persistent storage if needed
• Multi-region support

Prerequisites

• flyctl CLI installed and authenticated
• Fly.io account
• fly.toml in the app directory (or generated by catapulta)