Introduce typed TorrentSource for adding torrents

[artrixdotdev]

Problem

Engine::add_torrent currently accepts impl ToString and guesses source type with string prefixes like http and magnet:. A frontend should pass typed intent instead of relying on string heuristics.

Relevant code:

• crates/libtortillas/src/engine/mod.rs:254

Dependency

Blocked by #222 because TorrentSource should be part of the stable facade.

Proposal

Add a typed source enum, for example:

pub enum TorrentSource {
    Magnet(String),
    TorrentFilePath(PathBuf),
    TorrentFileBytes(Bytes),
    RemoteTorrentUrl(String),
}

Exact variants can differ, but source type should be explicit.

Acceptance criteria

• Adding a torrent no longer requires source detection from arbitrary strings.
• Invalid source/type combinations return typed errors.
• Existing path, magnet, and remote torrent behavior remains covered by tests.
• API docs show how a TUI should add torrents from user input.

[artrixdotdev]

Claiming this in branch feat/add-typed-torrent-sources.