feat(downloads): capture structured run stats and filtered logs at save time

Backend foundation for the upcoming Downloads modal refresh. Adds three
helpers on GalleryDLService:

  _compute_run_stats   exit_code + downloaded/skipped/per-item-failure/
                      warning counts, derived from stdout+stderr
  _extract_errors_warnings   filters stderr to just [error]/[warning] lines
                             so the modal can show a concise errors-only view
  _truncate_log        caps a log string (default 500KB) with head+tail+
                      elision marker, preventing verbose multi-hour runs
                      from bloating the download_sessions JSONB

Wires all three into download.metadata_ in the download task. No schema
change — everything nests into the existing metadata JSONB column.

Follow-up PR will consume these fields in the Downloads.vue modal.

backend/app/tasks/downloads.py

@@ -400,10 +400,21 @@ async def _download_source_async(source_id: int, download_id: int = None) -> dic
             # Update download record with results
             download.completed_at = utcnow()
             download.file_count = dl_result.files_downloaded
-            # Store full logs - cleanup task will age out old records
+            # Persist logs in three forms for the Downloads modal:
+            #   stdout/stderr: raw streams, capped via _truncate_log so huge
+            #     verbose runs don't balloon the JSONB row.
+            #   stderr_errors_warnings: pre-filtered [error]/[warning] lines so
+            #     the UI can show a concise errors-only panel by default.
+            #   run_stats: structured counts for the modal's header row.
+            run_stats = gdl_service._compute_run_stats(
+                dl_result.return_code, dl_result.stdout, dl_result.stderr
+            )
+            stderr_summary = gdl_service._extract_errors_warnings(dl_result.stderr)
             download.metadata_ = {
-                "stdout": dl_result.stdout if dl_result.stdout else None,
-                "stderr": dl_result.stderr if dl_result.stderr else None,
+                "stdout": gdl_service._truncate_log(dl_result.stdout) or None,
+                "stderr": gdl_service._truncate_log(dl_result.stderr) or None,
+                "stderr_errors_warnings": stderr_summary or None,
+                "run_stats": run_stats,
                 "duration_seconds": dl_result.duration_seconds,
             }