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>

frontend/templates/base.html

@@ -63,6 +63,7 @@
             <a href="/upload" class="text-gray-700 hover:text-gray-900" {% if request and request.url.path == '/upload' %}aria-current="page"{% endif %}>Upload</a>
             <a href="/files" class="text-gray-700 hover:text-gray-900" {% if request and request.url.path == '/files' %}aria-current="page"{% endif %}>Files</a>
             <a href="/search" class="text-gray-700 hover:text-gray-900" {% if request and request.url.path == '/search' %}aria-current="page"{% endif %}>Search</a>
+            <a href="/pipelines" class="text-gray-700 hover:text-gray-900" {% if request and request.url.path == '/pipelines' %}aria-current="page"{% endif %}>Pipelines</a>
             <a href="/pricing" class="text-gray-700 hover:text-gray-900" {% if request and request.url.path == '/pricing' %}aria-current="page"{% endif %}>Pricing</a>
 
             <!-- Admin dropdown (shown only for admin users via JS) -->
@@ -175,6 +176,7 @@
               <a href="/upload" class="block px-3 py-3 rounded-md text-base font-medium text-gray-700 hover:text-gray-900 hover:bg-gray-50">Upload</a>
               <a href="/files" class="block px-3 py-3 rounded-md text-base font-medium text-gray-700 hover:text-gray-900 hover:bg-gray-50">Files</a>
               <a href="/search" class="block px-3 py-3 rounded-md text-base font-medium text-gray-700 hover:text-gray-900 hover:bg-gray-50">Search</a>
+              <a href="/pipelines" class="block px-3 py-3 rounded-md text-base font-medium text-gray-700 hover:text-gray-900 hover:bg-gray-50">Pipelines</a>
               <a href="/pricing" class="block px-3 py-3 rounded-md text-base font-medium text-gray-700 hover:text-gray-900 hover:bg-gray-50">Pricing</a>
 
               <!-- Admin section in mobile menu (shown only for admin users via JS) -->