fix(api): let an artist download their own track with downloads off

[dylanjeffers]

Problem

Michael has flagged twice (June 25, today) that artists can't download their own tracks. Both reported tracks — kapyo x artikko - pasionaria (Rpd38wj) and Elequence - Novalunosis (r2Xp9aY) — have is_downloadable = false.

GET /v1/tracks/{id}/download serves track.Download, and dbv1.TracksKeyed only populates that field when rawTrack.IsDownloadable is true. There is no owner exception, so the edit page's Download File button — the one paired with Replace File, which exists precisely so an artist can retrieve what they uploaded — 404s for the owner and surfaces as the generic "Something went wrong. Please check your connection and storage and try again."

79 of 100 sampled trending tracks have downloads off, which matches the "most all tracks" in the report. The public download path itself is healthy: every sampled is_downloadable track returns 206 anonymously.

The original file is still there — GET /v1/tracks/Rpd38wj/inspect?original=true reports a 63 MB audio/wave blob on the content node. Nothing was lost; the API just refused to sign a link to it for the person who uploaded it.

Fix

When track.Download is nil, fall back to signing a link for a requester who proves — with the wallet signature already on the request — that they own the track or hold a grant on the account that does. Everyone else is unchanged: a stranger, signed or not, still gets the 404 the artist asked for by leaving downloads off.

Ownership is read from the recovered wallet rather than the user_id query param behind myId. user_id is the caller's own claim; it happens to be verified on this route today only because the route sits off authMiddleware's advisory-user_id allowlist, and an artist's original master should not depend on that list continuing to exclude it.

Also fixes the served filename for a case this change makes reachable: with no orig_file_cid the bytes are the mp3 transcode, so the name must not carry the uploaded file's extension — a .wav name on mp3 bytes is a file most editors refuse to open.

Tests

api/v1_track_download_test.go covers owner, manager-with-grant, anonymous, another wallet, and user_id spoofing both unsigned and signed by a stranger, plus the mp3 filename fallback. Full ./api/... suite passes (TestSearch needs Elasticsearch, unrelated).

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