feat(pipelines): add custom processing pipeline engine

- Add Pipeline and PipelineStep models with user-specific ownership
- Add pipeline_id FK column to FileRecord
- Migration 017_add_pipelines (batch mode for SQLite FK compat)
- Pipeline CRUD API at /api/pipelines with step management endpoints
- Reorder steps PUT endpoint placed before parameterised {step_id} routes
- POST /api/files/{id}/assign-pipeline for per-file pipeline assignment
- Admin-only POST /api/pipelines/admin/system for system-level pipelines
- Management UI at /pipelines (Jinja2 + Alpine.js + Tailwind)
- Pipelines link added to desktop and mobile navigation
- 41 new tests in tests/test_api_pipelines.py (all passing)

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

app/views/pipelines.py

@@ -0,0 +1,34 @@
+"""Pipelines view: management UI for processing pipelines."""
+
+import logging
+
+from fastapi import HTTPException, Request, status
+
+from app.views.base import APIRouter, require_login, settings, templates
+
+logger = logging.getLogger(__name__)
+router = APIRouter()
+
+
+@router.get("/pipelines")
+@require_login
+async def pipelines_page(request: Request):
+    """Processing pipeline management page for the current user.
+
+    Regular users manage their own pipelines.  Admins additionally have access
+    to system-level pipelines through the same UI.
+    """
+    try:
+        return templates.TemplateResponse(
+            "pipelines.html",
+            {
+                "request": request,
+                "app_version": settings.version,
+            },
+        )
+    except Exception as exc:
+        logger.error(f"Error loading pipelines page: {exc}")
+        raise HTTPException(
+            status_code=status.HTTP_500_INTERNAL_SERVER_ERROR,
+            detail="Failed to load pipelines page",
+        )