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/tests/test_save_session_as_tree.py

@@ -155,6 +155,7 @@ class TestSaveSessionAsTreeAPI:
         session = Session(
             tree_id=tree.id,
             user_id=UUID(test_user["user_data"]["id"]),
+            account_id=UUID(test_user["user_data"]["account_id"]),
             tree_snapshot=tree.tree_structure,
             path_taken=["root"],
             decisions=[{"node_id": "root", "timestamp": datetime.now(timezone.utc).isoformat()}],
@@ -199,6 +200,7 @@ class TestSaveSessionAsTreeAPI:
         session = Session(
             tree_id=tree.id,
             user_id=UUID(test_user["user_data"]["id"]),
+            account_id=UUID(test_user["user_data"]["account_id"]),
             tree_snapshot=tree.tree_structure,
             path_taken=["root"],
             decisions=[],
@@ -239,6 +241,7 @@ class TestSaveSessionAsTreeAPI:
         session = Session(
             tree_id=tree.id,
             user_id=UUID(test_user["user_data"]["id"]),
+            account_id=UUID(test_user["user_data"]["account_id"]),
             tree_snapshot=tree.tree_structure,
             path_taken=["root"],
             decisions=[],
@@ -279,6 +282,7 @@ class TestSaveSessionAsTreeAPI:
         session = Session(
             tree_id=tree.id,
             user_id=UUID(test_user["user_data"]["id"]),
+            account_id=UUID(test_user["user_data"]["account_id"]),
             tree_snapshot=tree.tree_structure,
             path_taken=["root"],
             decisions=[],
@@ -352,6 +356,7 @@ class TestSaveSessionAsTreeAPI:
         session = Session(
             tree_id=tree.id,
             user_id=other_user.id,
+            account_id=UUID(test_user["user_data"]["account_id"]),
             tree_snapshot=tree.tree_structure,
             path_taken=["root"],
             decisions=[],