fix: report files that could not be downloaded when the import finishes

[oleksandr-nc]

Closes #139

Final part, following #142 and #143: a file that failed to download even after the fresh-URL retry was dropped silently, and the completion notification counted it as imported — it reported totalSeenNumber, which increments before the download is attempted.

What changes

• Download failures are counted in a nb_failed_files user setting, accumulated across import batches the same way nb_imported_files already works, and reset at import start and finish. The names of the first ten failed files are kept as well.
• The finish notification now reports the number of files actually downloaded (the accumulated imported counter, instead of the last batch's seen count) and, when there were failures, appends: "N files could not be downloaded, check the server logs for details." — with the kept file names in the notification body: "Could not download: DSC02561.jpg, DSC02561_thumb.jpg, and 9 more". Users without server-log access see which files are missing; the full list is in the log (redacted since fix: bound stalled file downloads and keep download URLs out of the log #142).
• Files skipped because they already existed are counted too (one config write per listing page, since skips are frequent on re-imports) and reported: "1993 files were already there." Re-running an import into an already-imported folder used to end with a notification claiming everything was imported again; with the corrected imported-count alone it would have read "0 files were imported", which looks like a failure — now it explains itself.
• Distinguishing downloaded / existing / failed outcomes properly in getFile() also fixes a pre-existing quirk where a successfully imported empty file was never counted as imported.
• Notifications queued before an upgrade carry none of the new parameters and render exactly as before.

With this, all four problems from #139 are addressed: the indefinite hang (#142), the token leak into logs (#142), the expired listing URL (#143), and unreported missing files (this PR).

Tests

• The counters: an import over a mocked four-file drive (one success, one double-failure, one empty file, one already existing) asserts the failed/skipped/imported counters, the kept failed-file names, and the exact download attempt sequence.
• The finish path: a finishing job asserts the notification carries the accumulated counts and file names, and that all counters are reset afterwards.
• The notifier: rendered subject and body with zero, one, few and many failures (including the "and N more" overflow), the skipped-files sentence, the re-run case, the pre-upgrade notification without the new parameters, and the unknown app/subject error paths.

composer cs:check, psalm and the suite (22 tests) pass locally on NC 33 / PHP 8.3. All notification variants were additionally rendered live on a real NC 35 instance through the OCS API to verify the actual L10N output.