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/conftest.py

@@ -75,6 +75,19 @@ async def test_db() -> AsyncGenerator[AsyncSession, None]:
                 ('team', NULL, NULL, NULL, true, true, '["markdown", "text", "html"]')
         """))
 
+        # Seed the platform/system account (PLATFORM_ACCOUNT_ID) needed by
+        # global categories, gallery items, and other platform-owned content.
+        await conn.execute(sa.text("""
+            INSERT INTO accounts (id, name, display_code, created_at, updated_at)
+            VALUES (
+                '00000000-0000-0000-0000-000000000001',
+                'ResolutionFlow System',
+                'RF-SYS-1',
+                NOW(), NOW()
+            )
+            ON CONFLICT (id) DO NOTHING
+        """))
+
     # Create async session maker
     async_session_maker = async_sessionmaker(
         engine,