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/account.py

@@ -0,0 +1,39 @@
+from datetime import datetime
+from typing import Optional
+from uuid import UUID
+from pydantic import BaseModel, Field
+
+
+class AccountResponse(BaseModel):
+    id: UUID
+    name: str
+    display_code: str
+    owner_id: UUID
+    stripe_customer_id: Optional[str] = None
+    created_at: datetime
+    updated_at: datetime
+
+    model_config = {"from_attributes": True}
+
+
+class AccountUpdate(BaseModel):
+    name: Optional[str] = Field(None, min_length=1, max_length=255)
+
+
+class AccountInviteCreate(BaseModel):
+    email: str = Field(..., max_length=255)
+    role: str = Field("engineer", pattern="^(engineer|viewer)$")
+    expires_in_days: Optional[int] = Field(None, ge=1, le=30)
+
+
+class AccountInviteResponse(BaseModel):
+    id: UUID
+    account_id: UUID
+    email: str
+    code: str
+    role: str
+    expires_at: Optional[datetime] = None
+    created_at: datetime
+    used_at: Optional[datetime] = None
+
+    model_config = {"from_attributes": True}