feat(admin): extend /admin/plan-limits to manage plan_billing fields

Task 30 of self-serve signup Phase 2. Super-admins can now manage Stripe
IDs, display names, prices, and public/archived flags via the existing
admin plan-limits endpoints.

- GET /admin/plan-limits now outer-joins plan_billing and returns
  merged PlanLimitWithBillingResponse rows. Plans without a
  plan_billing row return None for the billing fields.
- PUT /admin/plan-limits accepts the new optional billing fields and
  upserts plan_billing in the same transaction. If no plan_billing
  row exists for the plan and the body includes any billing field, a
  row is created (display_name defaults to plan.capitalize() when
  omitted; display_name is never NULLed out on an existing row).
- After commit, the handler queries account_ids on the affected plan
  and calls BillingService.invalidate_billing_cache(account_ids).
  This is a no-op stub today (logs only) — there's no in-process
  billing cache yet. TODO comment marks the wire-up point.
- 3 new integration tests cover GET-with-billing-present, PUT creating
  a plan_billing row, and the invalidation hook being awaited with a
  list of account_ids.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/app/schemas/admin.py

@@ -172,6 +172,21 @@ class PlanLimitResponse(BaseModel):
         from_attributes = True
 
 
+class PlanLimitWithBillingResponse(PlanLimitResponse):
+    """PlanLimits + plan_billing fields merged. Billing fields are None when no
+    plan_billing row exists for the plan yet."""
+    display_name: Optional[str] = None
+    description: Optional[str] = None
+    monthly_price_cents: Optional[int] = None
+    annual_price_cents: Optional[int] = None
+    stripe_product_id: Optional[str] = None
+    stripe_monthly_price_id: Optional[str] = None
+    stripe_annual_price_id: Optional[str] = None
+    is_public: Optional[bool] = None
+    is_archived: Optional[bool] = None
+    sort_order: Optional[int] = None
+
+
 class PlanLimitUpdate(BaseModel):
     plan: str
     max_trees: Optional[int] = None
@@ -180,6 +195,19 @@ class PlanLimitUpdate(BaseModel):
     custom_branding: bool = False
     priority_support: bool = False
     export_formats: list = Field(default_factory=lambda: ["markdown", "text"])
+    # plan_billing fields — all optional, partial-update semantics. If any are
+    # set in the body, the admin endpoint upserts the plan_billing row in the
+    # same transaction.
+    display_name: Optional[str] = None
+    description: Optional[str] = None
+    monthly_price_cents: Optional[int] = None
+    annual_price_cents: Optional[int] = None
+    stripe_product_id: Optional[str] = None
+    stripe_monthly_price_id: Optional[str] = None
+    stripe_annual_price_id: Optional[str] = None
+    is_public: Optional[bool] = None
+    is_archived: Optional[bool] = None
+    sort_order: Optional[int] = None
 
 
 class AccountOverrideCreate(BaseModel):