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

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()

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

@@ -82,6 +82,11 @@ class TaskPipelineStateRow(Base):
     #: A waived test pass must always be explainable: the reason is stored,
     #: never inferred (PLAN-v2 §3, green gate).
     waiver_reason: Mapped[str] = mapped_column(Text, default="", nullable=False)
+    #: The latest accepted diff (the writer caps it). The worktree is gone after the
+    #: merge, so the task page reads the diff from here.
+    last_diff: Mapped[str] = mapped_column(Text, default="", nullable=False)
+    #: The sha the merge landed on, once the state is COMPLETED.
+    merged_sha: Mapped[str] = mapped_column(String(64), default="", nullable=False)
     parent_task_id: Mapped[str | None] = mapped_column(
         ForeignKey("tasks.id", ondelete="SET NULL"), nullable=True
     )

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

@@ -143,7 +143,7 @@ class OrgPolicyRow(Base):
     organization_id: Mapped[str] = mapped_column(
         ForeignKey("organizations.id", ondelete="CASCADE"), primary_key=True
     )
-    max_employees: Mapped[int] = mapped_column(Integer, default=5, nullable=False)
+    max_employees: Mapped[int] = mapped_column(Integer, default=7, nullable=False)
     #: ``None`` means "no explicit budget" (an empty/unlimited wallet view).
     monthly_credit_budget: Mapped[float | None] = mapped_column(Float, nullable=True)
     automation_enabled: Mapped[bool] = mapped_column(Boolean, default=True, nullable=False)