fix: propagate silently-swallowed errors in the sync engine

[devin-ai-integration]

Summary

Several Results in the sync engine were dropped with let _ = ... / .ok(), so DB and filesystem failures passed unnoticed — some of them affecting exactly the invariants the test suite guards (no silent data loss, conflicts surfaced). This propagates or at least logs them, matching patterns already used elsewhere in the same modules.

drive/sync.rs — full_sync (highest impact): inventory writes were swallowed inside the 3-way reconcile loop. full_sync already uses ? for every other inventory/API call, so these now do too:

- let _ = mount.inventory.batch_delete_by_path(vec![&path_str]);   // 3 delete branches
- let _ = mount.inventory.upsert(&entry);
- let _ = mount.inventory.mark_as_conflicted(&path_str, Pending);
+ mount.inventory.<op>(...).context("…")?;

The mark_as_conflicted case is the important one: if that write failed silently, the file was not frozen and could later be overwritten — the exact silent-data-loss scenario both_sides_modified_* / conflicted_file_stays_frozen assert against. tasks/upload.rs already surfaces this same call's error, so sync.rs was inconsistent.

tasks/queue.rs — re_enqueue_offline_tasks: update_task and dispatch_task errors were dropped while the task was still counted as re-enqueued. Now mirrors the sibling resume loop in the same file (warn + continue, only count on success), so a task whose offline_waiting flag couldn't be cleared isn't falsely reported as re-enqueued.

tasks/download.rs: when mirroring a remote deletion, a failed remove_file was .ok()'d and the task returned Ok(()) — the local file silently survived. Now logs a warning.

uploader/mod.rs: failure to delete an expired upload session was dropped; now logged (control flow unchanged — still best-effort).

Verification

cargo clippy -p cloudreve-sync --tests (no new warnings) and cargo test -p cloudreve-sync — all 55 tests pass.

Link to Devin session: https://app.devin.ai/sessions/baccafbde57044f9b0a6f246ca385ff5
Requested by: @cedev-1

[devin-ai-integration]

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