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/user.py

@@ -13,6 +13,7 @@ class UserBase(BaseModel):
 class UserCreate(UserBase):
     password: str = Field(..., min_length=10, description="Password must be at least 10 characters")
     invite_code: Optional[str] = Field(None, description="Invite code for registration (required when invite system is enabled)")
+    account_invite_code: Optional[str] = Field(None, description="Account invite code to join an existing account")
 
     @field_validator('password')
     @classmethod
@@ -38,11 +39,11 @@ class UserLogin(BaseModel):
 
 class UserResponse(UserBase):
     id: UUID
-    role: str
+    role: str = "engineer"
+    account_id: UUID
+    account_role: str
     is_super_admin: bool = False
-    is_team_admin: bool = False
     is_active: bool = True
-    team_id: Optional[UUID] = None
     created_at: datetime
     last_login: Optional[datetime] = None
 
@@ -54,5 +55,5 @@ class RoleUpdate(BaseModel):
     role: Literal["engineer", "viewer"]
 
 
-class TeamAdminUpdate(BaseModel):
-    is_team_admin: bool
+class AccountRoleUpdate(BaseModel):
+    account_role: str = Field(..., pattern="^(engineer|viewer)$")