feat: update all endpoints and schemas for account-based model

Replace team_id with account_id across all API endpoints (trees,
categories, tags, steps, step_categories, admin, auth). Add new
accounts and webhooks endpoints. Registration now atomically creates
Account + Subscription, with account_invite_code bypassing the
platform invite gate.

Schemas updated for account_id/account_role. 82 tests passing
including 18 new tests for accounts, subscriptions, and permissions.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/app/schemas/subscription.py

@@ -0,0 +1,40 @@
+from typing import Optional
+from uuid import UUID
+from datetime import datetime
+from pydantic import BaseModel
+
+
+class SubscriptionResponse(BaseModel):
+    id: UUID
+    plan: str
+    status: str
+    billing_interval: Optional[str] = None
+    current_period_start: Optional[datetime] = None
+    current_period_end: Optional[datetime] = None
+    cancel_at_period_end: bool = False
+    stripe_subscription_id: Optional[str] = None
+
+    model_config = {"from_attributes": True}
+
+
+class PlanLimitsResponse(BaseModel):
+    plan: str
+    max_trees: Optional[int] = None
+    max_sessions_per_month: Optional[int] = None
+    max_users: Optional[int] = None
+    custom_branding: bool = False
+    priority_support: bool = False
+    export_formats: list[str] = ["markdown", "text"]
+
+    model_config = {"from_attributes": True}
+
+
+class UsageResponse(BaseModel):
+    tree_count: int
+    session_count_this_month: int
+
+
+class SubscriptionDetails(BaseModel):
+    subscription: SubscriptionResponse
+    limits: PlanLimitsResponse
+    usage: UsageResponse