feat(integration): prepaid billing, engine UI, credits console, direct tasks, v2 final

- Prepaid credit system: org wallet, debit on usage, chat gate, admin credit console
- Engine UI: /manager, /tasks, /tasks/[id], /workforce, /dashboard, /traces pages
- Direct task creation (HTTP POST /tasks, MCP tasks_create) at pending_manager_review
- Pause/resume/cancel/answer actions in engine API and UI
- Employee cap (7 per org default), review pipeline guard (active reviewer required)
- Scheduler: wide window prevents org starvation; archived employees excluded from cap
- Tenant isolation: project-id cross-org 404, engine never uses another tenant's sandbox
- Order route fix: manager built with pipeline states + employee profiles
- Contract extension: ModelGrantPort.get_org_policy with DefaultOrgPolicy
- Engine event types in @agy/contracts (task.step, task.gate, run.log, run.tool, team.changed)
- api-engine.ts typed client; api-credit.ts client
- Alembic migration c4d5e6f7a8b9: prepaid credit + engine UI tables
- 859 tests passed, 1 skipped; ruff/mypy/tsc/eslint/vitest all green

Refs: PLAN-v2.md §9 (T1–T43), todo-v2-final.md

apps/api/app/infrastructure/db/models.py

@@ -1023,6 +1023,7 @@ class QualityMetricRow(Base):
 # parallel agents never edit the same file). Imported last so every table is
 # registered on ``Base.metadata`` before ``create_all`` / alembic autogenerate.
 # ---------------------------------------------------------------------------
+from app.infrastructure.db import models_billing as _models_billing  # noqa: E402,F401
 from app.infrastructure.db import models_delivery as _models_delivery  # noqa: E402,F401
 from app.infrastructure.db import models_engine as _models_engine  # noqa: E402,F401
 from app.infrastructure.db import models_org as _models_org  # noqa: E402,F401