fix(pipelines): seed standard processing pipeline as system default on startup

The pipeline management UI showed an empty list after first boot because no
default system pipeline was created. This adds seed_default_pipeline() which:

- Creates a system-owned (owner_id=NULL), is_default=True pipeline named
  "Standard Processing Pipeline" at application startup
- Steps mirror the current hardcoded Celery processing workflow:
  convert_to_pdf → check_duplicates → ocr → extract_metadata →
  embed_metadata → compute_embedding → send_to_destinations
- Is idempotent: no-op if any system pipeline already exists
- Handles missing pipelines table gracefully (during first migration run)

Also wires the seeder into app/main.py lifespan startup using the same
pattern as seed_default_plans.

9 new tests added covering creation, step order, idempotency, and API visibility.

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

app/main.py

@@ -112,6 +112,20 @@ async def lifespan(app: FastAPI):
     except Exception:
         logging.debug("Subscription plan seeding skipped — DB may not be ready yet")  # noqa: S110
 
+    # Seed the default system pipeline (mirrors the current hardcoded processing
+    # workflow) so it is immediately visible in the Pipelines management UI.
+    try:
+        from app.api.pipelines import seed_default_pipeline as _seed_pipeline
+        from app.database import SessionLocal as _SessionLocal  # noqa: F811 (re-import for clarity)
+
+        _db_pipeline = _SessionLocal()
+        try:
+            _seed_pipeline(_db_pipeline)
+        finally:
+            _db_pipeline.close()
+    except Exception:
+        logging.debug("Default pipeline seeding skipped — DB may not be ready yet")  # noqa: S110
+
     # Application is now running
     yield