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/api/__init__.py

@@ -17,6 +17,7 @@ from app.api.google_drive import router as google_drive_router
 from app.api.logs import router as logs_router
 from app.api.onedrive import router as onedrive_router
 from app.api.openai import router as openai_router
+from app.api.pipelines import router as pipelines_router
 from app.api.plans import router as plans_router
 from app.api.process import router as process_router
 from app.api.queue import router as queue_router
@@ -60,3 +61,4 @@ router.include_router(webhooks_router)
 router.include_router(database_router)
 router.include_router(subscriptions_router)
 router.include_router(plans_router)
+router.include_router(pipelines_router)