feat(refactor/phase1): foundation infrastructure for modular pipeline

Phase 1 of PLAN_REFACTOR.md — all four sub-tasks implemented:

1.1 PipelineLogger (backend/app/core/pipeline_logger.py)
  - Structured step_start/step_done/step_error/step_progress API
  - Publishes to Python logging AND Redis SSE via log_task_event
  - Context manager `pl.step("name")` for auto-timing

1.2 RenderJobDocument (backend/app/domains/rendering/job_document.py)
  - Pydantic JSONB schema: state machine + per-step records + timing
  - begin_step/finish_step/fail_step/skip_step helpers
  - Migration 048: adds render_job_doc JSONB column to order_lines
  - OrderLine model updated with render_job_doc field

1.3 TenantContextMiddleware (backend/app/core/middleware.py)
  - Decodes JWT, stores tenant_id + role in request.state
  - get_db updated to auto-apply RLS SET LOCAL from request.state
  - Registered in main.py (runs before every request)
  - JWT now embeds tenant_id claim via create_access_token()
  - Login endpoint passes tenant_id to token creation

1.4 ProcessStep Registry (backend/app/core/process_steps.py)
  - StepName StrEnum with all 20 pipeline step names
  - Single source of truth for log prefixes, DB records, UI labels

Also adds db_utils.py with set_tenant_sync() + get_sync_session()
for use inside Celery tasks (bypass-safe RLS helper).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/domains/orders/models.py

@@ -131,6 +131,7 @@ class OrderLine(Base):
     render_status: Mapped[str] = mapped_column(String(20), nullable=False, default="pending")
     result_path: Mapped[str | None] = mapped_column(String(1000), nullable=True)
     render_log: Mapped[dict | None] = mapped_column(JSONB, nullable=True)
+    render_job_doc: Mapped[dict | None] = mapped_column(JSONB, nullable=True)
     ai_validation_status: Mapped[str] = mapped_column(String(20), nullable=False, default="not_started")
     ai_validation_result: Mapped[dict | None] = mapped_column(JSONB, nullable=True)
     flamenco_job_id: Mapped[str | None] = mapped_column(String(100), nullable=True)