feat(slicer): Pipelines — multi-copy + class targeting + fanout + runs dashboard + retry-failed + WS updates (#1425 PR C — completes the v3 design)

PR A/B turned the slice modal's preset bundle into a one-click dispatch
with a pinned target printer. PR C closes the original issue: operators
type in a number of copies, Bambuddy slices once and distributes prints
across a fleet per the pipeline's chosen fanout strategy. A new dashboard
surfaces every run with filters, expandable per-copy status, cancel,
and retry-failed-copies. WS pushes keep everything live.

Backend
- copies field on POST /run, capped by new pipeline_max_copies setting
  (default 50, hard cap 1000). PipelineRun.parent_run_id chains retries.
- SlicerPipelineUpdate accepts target_kind (specific_printer /
  printer_class), target_model_class, fanout_strategy.
- Eligibility matcher branches: class-targeting enumerates matching
  Printer rows, runs per-printer checks via a status_lookup closure,
  returns printer_reports[]. New issue kinds: no_class_matches,
  class_not_set.
- _pick_assignments distributes copies per strategy:
  - max_parallel: target_model set, printer_id None — scheduler picks
  - round_robin: copy i → eligible[i % N], fixed printer_id
  - fill_one_first: all copies pinned to eligible[0]
  All three reuse the slice-once path through slice_dispatch.enqueue.
- New routes:
  - GET /pipeline-runs (paginated, filterable by pipeline + status)
  - POST /pipeline-runs/{id}/retry-failed (creates child run with
    copies = failed+cancelled count, parent_run_id set)
  - Cancel cascades to all N queue entries (only pending/queued)
- _roll_up_run_status computes run-level status from per-job statuses;
  introduces partial_failure for "some completed, some failed".
- ws_manager.broadcast_to_user emits pipeline_run_updated on every
  state transition with the full materialised response.

Frontend
- Pipeline editor: target_kind radio + class picker (filtered to
  installed models) + fanout-strategy radio. Read-only row shows
  "X1C · Round robin" for class pipelines.
- RunWithPipelineModal: copies number input bounded by
  settings.pipeline_max_copies. Accepts class-targeted pipelines.
- Settings → Workflow → Queue & Dispatch: new "Slicer Pipeline limits"
  card with the max-copies input.
- New /pipelines/runs dashboard page (sidebar entry, gated on
  pipelines:read). Two-filter dropdown, 25-per-page pagination, per-row
  expandable to job list, Cancel + Retry-failed buttons.
- useWebSocket case for pipeline_run_updated invalidates both
  pipeline-runs-all and pipeline-runs/{id} query keys.

frontend/src/__tests__/components/RunWithPipelineModal.test.tsx

@@ -131,6 +131,7 @@ describe('RunWithPipelineModal', () => {
         1,
         { kind: 'libraryFile', id: 99 },
         false,
+        1,
       );
       expect(onClose).toHaveBeenCalled();
     });
@@ -165,6 +166,7 @@ describe('RunWithPipelineModal', () => {
         1,
         { kind: 'archive', id: 7 },
         false,
+        1,
       );
     });
   });
@@ -202,7 +204,7 @@ describe('RunWithPipelineModal', () => {
     // Clicking Run anyway fires the run with force=true and the new source-kind shape.
     await user.click(screen.getByRole('button', { name: /Run anyway/i }));
     await waitFor(() => {
-      expect(mockApi.runPipeline).toHaveBeenCalledWith(1, { kind: 'libraryFile', id: 99 }, true);
+      expect(mockApi.runPipeline).toHaveBeenCalledWith(1, { kind: 'libraryFile', id: 99 }, true, 1);
     });
   });
 });