fix: propagate account_id through all write paths missing NOT NULL coverage

Service layer (production code):
- branch_manager: set account_id on SessionBranch (root + fork) and ForkPoint
  from session.account_id; load session in create_fork for this purpose
- handoff_manager: set account_id on SessionHandoff from session.account_id
- ai_suggestions endpoint: set account_id on AISuggestion from current_user
- steps endpoint (/feedback): set account_id on StepRating from current_user
- ratings endpoint: set account_id on StepRating from current_user

Test infrastructure:
- conftest.py: seed PLATFORM_ACCOUNT_ID (00000000-...-0001) account after
  Base.metadata.create_all so global categories and gallery items have a valid FK
- test_rls_isolation: add _ensure_rls_schema fixture that runs
  'alembic upgrade head' before module tests — previous function-scoped
  test_db fixtures drop the schema, leaving the RLS tests with no tables
- test_branding: create Account before User in helper functions
- test_admin_gallery: set account_id=PLATFORM_ACCOUNT_ID on Tree/ScriptTemplate
- test_public_templates: set account_id=PLATFORM_ACCOUNT_ID on Tree,
  ScriptTemplate, TreeCategory
- test_resolution_outputs: set account_id=session.account_id on
  SessionResolutionOutput
- test_analytics_phase5: set account_id on PsaPostLog
- test_draft_trees: replace account_id=None with PLATFORM_ACCOUNT_ID in
  migration default test (NOT NULL now enforced)
- test_maintenance_schedules: set account_id on other_tree
- test_save_session_as_tree: set account_id on all 5 Session() constructors

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

backend/app/api/endpoints/ai_suggestions.py

@@ -43,6 +43,7 @@ async def create_suggestion(
     suggestion = AISuggestion(
         tree_id=data.tree_id,
         user_id=current_user.id,
+        account_id=current_user.account_id,
         session_id=data.session_id,
         action_type=data.action_type,
         target_node_id=data.target_node_id,

backend/app/api/endpoints/ratings.py

@@ -91,6 +91,7 @@ async def submit_step_feedback(
         new_rating = StepRating(
             step_id=step_id,
             user_id=current_user.id,
+            account_id=current_user.account_id,
             session_id=session_uuid,
             was_helpful=data.was_helpful,
             # rating is nullable now — thumbs-only mode

backend/app/api/endpoints/steps.py

@@ -460,6 +460,7 @@ async def rate_step(
     rating = StepRating(
         step_id=step_id,
         user_id=current_user.id,
+        account_id=current_user.account_id,
         rating=rating_data.rating,
         was_helpful=rating_data.was_helpful,
         review_text=rating_data.review_text,