New Aria2 JSON-RPC adapter, alongside qBittorrent / Transmission / Deluge. Aria2's
RPC (default :6800/jsonrpc) maps cleanly onto the uniform adapter contract:
- the --rpc-secret token leads every call as "token:<secret>" (no username — the
secret uses the existing password field),
- addUri returns a GID (our torrent id); tellStatus → TorrentStatus with state
mapping (active→downloading, or seeding once the payload is complete; waiting→
queued; etc.),
- remove picks forceRemove vs removeDownloadResult by status, and (since aria2
doesn't delete files on remove) unlinks the file paths itself for delete_files,
- bare-host URLs get /jsonrpc appended.
Wired into adapter_for_type + the Settings dropdown (with a help note: port 6800,
secret in the Password field). All adapter methods go through the same interface,
so the stall/orphan handling and downloads pipeline work unchanged.
Tests (9): registry wiring, state mapping (incl. active→seeding), token-prefixed
params, /jsonrpc fixup, status parse (+ name fallback, no div-by-zero). 126 torrent
tests green; ruff clean.
