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/step_library.py

@@ -30,7 +30,7 @@ class StepLibraryBase(BaseModel):
 
 
 class StepLibraryCreate(StepLibraryBase):
-    team_id: Optional[UUID] = None
+    account_id: Optional[UUID] = None
 
 
 class StepLibraryUpdate(BaseModel):
@@ -45,7 +45,7 @@ class StepLibraryUpdate(BaseModel):
 class StepLibraryResponse(StepLibraryBase):
     id: UUID
     created_by: UUID
-    team_id: Optional[UUID] = None
+    account_id: Optional[UUID] = None
     usage_count: int
     rating_average: Decimal
     rating_count: int