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/domain/billing/models.py

@@ -83,6 +83,34 @@ class CreditWallet:
         self.updated_at = utcnow()
 
 
+class CreditKind(StrEnum):
+    """Why a wallet's balance moved."""
+
+    OWNER_CREDIT = "owner_credit"
+    STARTING_CREDIT = "starting_credit"
+    USAGE = "usage"
+    ADJUSTMENT = "adjustment"
+
+
+@dataclass(slots=True, kw_only=True)
+class CreditTransaction:
+    """One change to a wallet's balance, with the balance it left behind.
+
+    ``amount`` is signed: a credit is positive, a debit is negative.
+    ``usage_event_id`` is set on usage debits only, so one usage row debits once.
+    """
+
+    organization_id: str
+    kind: CreditKind
+    amount: float
+    balance_after: float
+    reason: str = ""
+    actor_user_id: str | None = None
+    usage_event_id: str | None = None
+    id: str = field(default_factory=new_id)
+    created_at: datetime = field(default_factory=utcnow)
+
+
 @dataclass(slots=True, kw_only=True)
 class UsageEvent:
     """One billable unit of work. Append-only."""