fix(compression): reclaim failed originals instead of leaking them

Round 1 stopped the compression worker deleting an upload's original on failure —
a transient ENOSPC or a codec panic must never destroy the only copy of a photo a
guest cannot retake. But it left `Upload::soft_delete`'s quota refund in place, so
the bytes stayed on disk while the uploader was charged nothing for them.

That is worse than it first looks. The row is soft-deleted, so the file is
invisible and unowned; a guest hitting a reproducible codec failure can accumulate
orphans indefinitely at zero personal cost. And `active_uploaders` counts only
users with non-deleted uploads, so dropping out of that count RAISES everyone's
per-user ceiling — the leak loosens the very quota meant to contain it.

Keep the refund: the uploader didn't cause the failure and shouldn't silently lose
quota to it. Bound the leak instead, with an hourly sweep alongside the existing
session cleanup in `spawn_periodic_tasks`, reclaiming failed originals older than
14 days — comfortably longer than any single event, so an operator investigating a
failed upload still has the file.

The selection predicate is the entire safety argument, so it is deliberately
narrow: `compression_status = 'failed'` AND soft-deleted AND past the window AND
`original_path <> ''`. That is exactly the state the give-up path leaves behind,
and it cannot reach a live upload, an owner-deleted one, or a failure still inside
its recovery window. `original_path` is cleared after a successful reclaim, which
makes the sweep idempotent — otherwise a row whose file is already gone is
re-selected on every tick forever. The row itself is kept as the audit trail.

Tests reproduce the selection verbatim (same pattern as upload_concurrency) and
assert it against five near-misses that must survive, both sides of the retention
boundary, and the idempotence property.

Also fixes two comments in export.rs still claiming "the compression worker
hard-deletes an original when its transcode fails" — no longer true, and the
defensive handling they justify is now justified by this sweep and by ordinary
deletes instead.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

backend/src/main.rs

@@ -70,6 +70,7 @@ async fn main() -> Result<()> {
         pool,
         state.rate_limiter.clone(),
         state.sse_tickets.clone(),
+        config.media_path.clone(),
     );
 
     // Ensure media directories exist