feat: add tenant_filter() helper and get_tenant_context dependency

tenant_filter(model, account_id) is the canonical app-layer tenant
scoping expression. Every query on a tenant table must use it.
build_tree_access_filter and build_step_visibility_filter updated
to call tenant_filter() internally for the account_id match.

get_tenant_context is a FastAPI dependency that returns account_id
or raises 403 if the user has no account — prevents raw access to
current_user.account_id and centralises the null check.

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

backend/app/api/deps.py

@@ -190,3 +190,20 @@ async def get_plan_limits_for_user(
     """Get plan limits for the current user's account."""
     from app.core.subscriptions import get_user_plan_limits
     return await get_user_plan_limits(current_user.account_id, db)
+
+
+async def get_tenant_context(
+    current_user: Annotated[User, Depends(get_current_active_user)],
+) -> UUID:
+    """Return the current user's account_id.
+
+    Use this dependency instead of reading current_user.account_id directly.
+    Raises 403 if the user has no account association (should not happen in
+    normal flows — users are always associated with an account on registration).
+    """
+    if current_user.account_id is None:
+        raise HTTPException(
+            status_code=status.HTTP_403_FORBIDDEN,
+            detail="User not associated with any account",
+        )
+    return current_user.account_id

backend/app/core/filters.py

@@ -1,10 +1,12 @@
 """
 Centralized query filters for ResolutionFlow.
 
-Provides reusable SQLAlchemy filter builders for tree access control
-and step visibility, used across multiple endpoint modules.
+Provides reusable SQLAlchemy filter builders for tree access control,
+step visibility, and the canonical tenant_filter used by all queries
+on tenant-scoped tables.
 """
 from __future__ import annotations
+import uuid
 from typing import TYPE_CHECKING
 
 from sqlalchemy import or_, and_, true as sa_true
@@ -13,6 +15,18 @@ if TYPE_CHECKING:
     from app.models.user import User
 
 
+def tenant_filter(model, account_id: uuid.UUID):
+    """Primary app-layer tenant filter.
+
+    MUST be used in every SELECT/UPDATE/DELETE on tenant tables.
+    RLS (Phase 2) is the safety net — this is the primary enforcement.
+
+    Usage:
+        stmt = select(Tree).where(tenant_filter(Tree, current_user.account_id), ...)
+    """
+    return model.account_id == account_id
+
+
 def build_tree_access_filter(current_user: User):
     """Build the access filter for trees based on user permissions.
 
@@ -36,10 +50,11 @@ def build_tree_access_filter(current_user: User):
         Tree.author_id == current_user.id,
     ]
     if current_user.account_id:
+        # Team-visible trees: use tenant_filter as the account match
         conditions.append(
             and_(
                 Tree.visibility == 'team',
-                Tree.account_id == current_user.account_id
+                tenant_filter(Tree, current_user.account_id),
             )
         )
     return or_(*conditions)
@@ -58,11 +73,14 @@ def build_step_visibility_filter(current_user: User):
     if current_user.account_id:
         return or_(
             StepLibrary.visibility == 'public',
-            and_(StepLibrary.visibility == 'team', StepLibrary.account_id == current_user.account_id),
-            StepLibrary.created_by == current_user.id  # Own private steps
+            and_(
+                StepLibrary.visibility == 'team',
+                tenant_filter(StepLibrary, current_user.account_id),
+            ),
+            StepLibrary.created_by == current_user.id,
         )
     else:
         return or_(
             StepLibrary.visibility == 'public',
-            StepLibrary.created_by == current_user.id
+            StepLibrary.created_by == current_user.id,
         )