First lift in the new PR6 batch. Pulls the 312-line candidate-fallback
download dispatcher out of `web_server.py` into a new module under the
existing `core/downloads/` package. Pure 1:1 lift — wrapper keeps the
original entry-point name so all callers (search/match pipeline) work
unchanged.
What `attempt_download_with_candidates` does:
1. Sort candidates by descending confidence.
2. For each candidate:
- Cancellation gates (3 points: top of loop, before download starts,
after download_id is assigned).
- Skip already-tried sources via the per-task `used_sources` set.
- Skip blacklisted sources (user-flagged bad matches).
- Race protection: bail when the task already has an active
download_id.
- `update_task_status('downloading')`, then `soulseek_client.download`.
3. On a successful download_id:
- Build `matched_downloads_context` entry keyed by
`make_context_key(username, filename)`.
- For tracks with clean Spotify metadata, pull track_number /
disc_number from (1) track_info → (2) track object → (3) Spotify
API call. When local album context is incomplete, the API response
backfills release_date / album_type / total_tracks / images / id.
- Set `is_album_download` based on explicit context flag or
heuristic (album differs from title, isn't "Unknown Album").
- Store task/batch IDs and track_info on the context for post-
processing + playlist-folder mode.
4. On a cancellation that wins the race after the download started:
- `cancel_download(...)` to stop the in-flight Soulseek transfer.
- `on_download_completed(batch_id, task_id, success=False)` to free
the worker slot.
5. On exception or download-start failure: reset task status to
'searching', continue to next candidate.
Dependencies injected via `CandidatesDeps` (7 fields) — soulseek_client,
spotify_client, run_async, get_database, update_task_status,
make_context_key, on_download_completed.
Diff vs original after `deps.X` → global X normalization is **zero
differences** — 312 lines orig = 312 lines lifted, byte-identical body
(including all whitespace, comments, log strings).
Tests: 14 new under tests/downloads/test_downloads_candidates.py
covering happy path (first candidate succeeds, confidence ordering),
used_sources dedup, blacklist skip, cancellation gates (cancelled
status, deleted task, active download_id, mid-flight cancel + cleanup
callback), failure paths (all candidates failed, exception during
download falls through to next), context payload (explicit album
context, track_number priority order, API backfill of incomplete album
metadata), and equal-confidence stable order.
Pre-existing behavior documented in tests:
`spotify_album_context['id']` initializes to a non-empty placeholder
'from_sync_modal' in the fallback path, so the API-backfill condition
`if not spotify_album_context.get('id')` never fires for the id field
specifically. Other album fields (release_date, album_type) backfill
fine because they default to empty.
Full suite: 1290 passing (was 1276). Ruff clean.
1c43ca2eef