Support per-app deploy targets (mixed providers)

[dannywillems]

Problem

Catapulta currently deploys all apps to a single VPS. In practice,
a stack often has services that belong on different providers:

• Static frontends -> Cloudflare Pages (free CDN)
• Lightweight APIs -> Fly.io (free tier)
• Stateful services -> VPS (needs disk, database)

There is no way to express this in catapulta today.

Proposal

Allow each app to specify its own deploy target/provider:

// Example: mixed deployment
let cdn_app = App::new("scribe-web")
    .provider(Provider::CloudflarePages { project: "scribe-web" })
    .build_cmd("npm run build")
    .build_dir("build");

let free_app = App::new("notifier")
    .provider(Provider::Fly { app: "my-notifier", region: "cdg" })
    .dockerfile("Dockerfile");

let vps_app = App::new("papyrus")
    .provider(Provider::Vps)  // current default behavior
    .dockerfile("Dockerfile");

The deploy command would dispatch each app to its respective
provider's deployer.

Benefits

• Optimize cost: free tiers for simple services, VPS only for
  what needs it
• Reduce VPS resource pressure
• Each service gets the right infrastructure for its needs

Related

• Support Cloudflare Pages as a deploy target for static sites #17 (Cloudflare Pages support)
• Support Fly.io as a deploy target for containers #18 (Fly.io support)