test(downloader): backend-execution integration tests

[mads-jm]

Only parsers/mappers are tested; the actual yt-dlp / spotDL / Spytify CLI invocation is untested. Add integration tests (fixtures / recorded output) for each backend.

Plan & context: WhatNext - docs/06 reports/report-260627-issue-reconciliation.md and report-260627-mvp-state-of-the-union.md.

[mads-jm]

Spec drafting surfaced a latent capability bug to fix alongside these tests: SpotdlBackend.supportedInputs declares 'spotify-id' (singular, from InputType in service/downloader/backends/backend.ts) but DownloadInput.type is 'url' | 'spotify-ids' (plural) and _inputToUrls matches 'spotify-ids' — so the declared capability can never be selected. Small fix(downloader) to ride with this. See epic-audio-acquisition-hardening.md.