feat(files): show assigned pipeline info on file status and detail views

The file detail page (/files/{id}/detail) and file view page (/files/{id})
previously showed no information about which processing pipeline was used.

Changes:
- _STEP_TYPE_TO_STAGES mapping: pipeline step_type → Celery log stage keys
  (with maintenance comment requiring updates when new step types are added)
- _ALWAYS_SHOW_STAGES: stages always visible regardless of pipeline
- _resolve_pipeline(db, file_record): resolves the pipeline for a file —
  uses explicit pipeline_id when set, falls back to active system default
- _compute_processing_flow: new pipeline_steps parameter; when provided,
  filters flow graph to only show stages for the pipeline's enabled steps
  (+ always-show stages + any stage that actually ran). Also adds
  convert_to_pdf to the flow stage catalogue.
- file_detail_page: passes pipeline_info + pipeline-filtered flow_data
- file_view_page: passes pipeline_info

Templates:
- file_detail.html: 'Processing Pipeline' detail row with name link and
  colour-coded badge (System Default / System / Custom)
- file_view.html: 'Pipeline' info row in sidebar with (default)/(custom) tag

Tests:
- TestPipelineInfoInViews with 14 tests covering _resolve_pipeline,
  _compute_processing_flow filtering, completeness assertion for
  _STEP_TYPE_TO_STAGES, and HTTP-level view tests

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

frontend/templates/file_view.html

@@ -301,6 +301,21 @@
           <span class="info-key">Hash</span>
           <span class="info-val" style="font-size:0.7rem;">{{ file.filehash[:32] }}…</span>
         </div>
+        <div class="info-row">
+          <span class="info-key">Pipeline</span>
+          <span class="info-val">
+            {% if pipeline_info %}
+            <a href="/pipelines" style="text-decoration:none;color:inherit;font-weight:500;" aria-label="View pipeline management for {{ pipeline_info.name }}">{{ pipeline_info.name }}</a>
+            {% if not pipeline_info.is_explicit %}
+            <span style="font-size:0.7em;color:#7c3aed;" title="System default pipeline applied automatically">(default)</span>
+            {% elif not pipeline_info.is_system %}
+            <span style="font-size:0.7em;color:#059669;" title="Custom user-defined pipeline">(custom)</span>
+            {% endif %}
+            {% else %}
+            <span style="color:#9ca3af;font-style:italic;">Standard</span>
+            {% endif %}
+          </span>
+        </div>
       </div>
 
       <!-- Actions -->