Stream the source files an imported AAF sequence references, over SSE.

asset_id is a PROJECT_ORIGINAL asset. Events, in order:

• cached — the last computed listing, if any (e.g. from preprocess, or an earlier call here), sent immediately so a caller has something to show before the AAF is reparsed.
• refreshed — the freshly recomputed listing (same shape as cached). Sent even when the sequence was already converted: a conversion does not have to cover every source, so the listing stays useful afterwards (link the sources that were missing, then convert again). The single case where the AAF is not reparsed is a cached listing that already links every source and a project that already exists — there is then nothing left to resolve and nothing to create, so only cached and project_ready are sent.
• project_ready (project_id) — last, when a project exists for this sequence: the one already converted (auto- or user-triggered), or one created right here because this fresh parse found every source resolved and nothing had been converted yet. An already-converted sequence is never converted again on a plain read — use POST .../convert for that.

Each source comes back either linked (asset_id set — validated by a user, or matched on the MXF material package UMID or a single full-path hit), with candidates to choose from when only the file name matched, or with neither when the file has not been imported at all. Choices saved through PUT .../sources win over the automatic match and come back as match: "user".