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_billing.py

@@ -0,0 +1,50 @@
+"""ORM tables of the prepaid credit ledger (v2).
+
+Use ``from app.core.database import Base`` and the helpers in
+``app.infrastructure.db.columns`` (``pk()``, ``created()``). Do NOT import from
+``app.infrastructure.db.models`` here: that module imports this one (import cycle).
+Foreign keys to existing tables are written as plain strings, e.g.
+``ForeignKey("organizations.id", ondelete="CASCADE")``.
+
+``credit_transactions`` is the history of an organization's prepaid wallet
+(``credit_wallets.balance``). Every change to the balance writes one row here, in
+the same transaction, with the balance it left behind:
+
+* ``owner_credit``, ``starting_credit`` and ``adjustment`` move the balance by a
+  signed amount (credit positive, debit negative);
+* ``usage`` is the debit for one usage row. ``usage_event_id`` is unique, so one
+  usage row debits the wallet at most once, even when it is written twice.
+"""
+
+from __future__ import annotations
+
+from datetime import datetime
+
+from sqlalchemy import Float, ForeignKey, Index, String, UniqueConstraint
+from sqlalchemy.orm import Mapped, mapped_column
+
+from app.core.database import Base
+from app.infrastructure.db.columns import created, pk
+
+
+class CreditTransactionRow(Base):
+    __tablename__ = "credit_transactions"
+    __table_args__ = (
+        Index("ix_credit_tx_org_created", "organization_id", "created_at"),
+        UniqueConstraint("usage_event_id", name="uq_credit_transactions_usage_event_id"),
+    )
+
+    id: Mapped[str] = pk()
+    organization_id: Mapped[str] = mapped_column(
+        ForeignKey("organizations.id", ondelete="CASCADE"), nullable=False, index=True
+    )
+    #: owner_credit | starting_credit | usage | adjustment (see ``CreditKind``).
+    kind: Mapped[str] = mapped_column(String(16), nullable=False)
+    #: Signed: a credit is positive, a debit is negative.
+    amount: Mapped[float] = mapped_column(Float, nullable=False)
+    balance_after: Mapped[float] = mapped_column(Float, nullable=False)
+    reason: Mapped[str] = mapped_column(String(500), default="", nullable=False)
+    actor_user_id: Mapped[str | None] = mapped_column(String(32), nullable=True)
+    #: The usage row this debit paid for. Unique: the idempotency of a usage debit.
+    usage_event_id: Mapped[str | None] = mapped_column(String(32), nullable=True)
+    created_at: Mapped[datetime] = created()