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/api/endpoints/steps.py

@@ -55,10 +55,10 @@ async def get_step_or_404(
 
 def build_visibility_filter(user: User):
     """Build SQLAlchemy filter for step visibility based on user."""
-    if user.team_id:
+    if user.account_id:
         return or_(
             StepLibrary.visibility == 'public',
-            and_(StepLibrary.visibility == 'team', StepLibrary.team_id == user.team_id),
+            and_(StepLibrary.visibility == 'team', StepLibrary.account_id == user.account_id),
             StepLibrary.created_by == user.id  # Own private steps
         )
     else:
@@ -249,7 +249,7 @@ async def get_step(
         "tags": step.tags,
         "visibility": step.visibility,
         "created_by": step.created_by,
-        "team_id": step.team_id,
+        "account_id": step.account_id,
         "usage_count": step.usage_count,
         "rating_average": step.rating_average,
         "rating_count": step.rating_count,
@@ -296,10 +296,10 @@ async def create_step(
         if not cat_result.scalar_one_or_none():
             raise HTTPException(status_code=400, detail="Invalid category")
 
-    # Team validation: can only set team_id to own team
-    team_id = step_data.team_id
-    if team_id and team_id != current_user.team_id and not current_user.is_super_admin:
-        raise HTTPException(status_code=403, detail="Cannot create step for another team")
+    # Account validation: can only set account_id to own account
+    account_id = step_data.account_id
+    if account_id and account_id != current_user.account_id and not current_user.is_super_admin:
+        raise HTTPException(status_code=403, detail="Cannot create step for another account")
 
     step = StepLibrary(
         title=step_data.title,
@@ -309,7 +309,7 @@ async def create_step(
         tags=step_data.tags,
         visibility=step_data.visibility,
         created_by=current_user.id,
-        team_id=team_id or current_user.team_id,
+        account_id=account_id or current_user.account_id,
     )
 
     db.add(step)
@@ -326,7 +326,7 @@ async def create_step(
         "tags": step.tags,
         "visibility": step.visibility,
         "created_by": step.created_by,
-        "team_id": step.team_id,
+        "account_id": step.account_id,
         "usage_count": step.usage_count,
         "rating_average": step.rating_average,
         "rating_count": step.rating_count,
@@ -393,7 +393,7 @@ async def update_step(
         "tags": step.tags,
         "visibility": step.visibility,
         "created_by": step.created_by,
-        "team_id": step.team_id,
+        "account_id": step.account_id,
         "usage_count": step.usage_count,
         "rating_average": step.rating_average,
         "rating_count": step.rating_count,