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_public_templates.py

@@ -11,6 +11,8 @@ from sqlalchemy.ext.asyncio import AsyncSession
 from app.models.script_template import ScriptCategory, ScriptTemplate
 from app.models.tree import Tree
 
+_PLATFORM_ACCOUNT_ID = uuid.UUID("00000000-0000-0000-0000-000000000001")
+
 
 # ---------------------------------------------------------------------------
 # Helpers
@@ -41,6 +43,7 @@ async def _create_featured_tree(db: AsyncSession, name: str = "Featured Flow", f
         description="A featured flow for the gallery",
         tree_type="troubleshooting",
         tree_structure=_make_tree_structure(4),
+        account_id=_PLATFORM_ACCOUNT_ID,
         is_gallery_featured=featured,
         is_active=True,
         usage_count=42,
@@ -74,6 +77,7 @@ async def _create_featured_script(
 ) -> ScriptTemplate:
     script = ScriptTemplate(
         category_id=category.id,
+        account_id=_PLATFORM_ACCOUNT_ID,
         name=name,
         slug=name.lower().replace(" ", "-"),
         description="A gallery-featured script",
@@ -312,7 +316,7 @@ class TestCategoriesEndpoint:
         from app.models.category import TreeCategory
 
         # Create a category and a featured tree in that category
-        cat = TreeCategory(name="Networking", slug="networking", is_active=True)
+        cat = TreeCategory(name="Networking", slug="networking", is_active=True, account_id=_PLATFORM_ACCOUNT_ID)
         test_db.add(cat)
         await test_db.commit()
         await test_db.refresh(cat)
@@ -321,6 +325,7 @@ class TestCategoriesEndpoint:
             name="Router Diagnostics",
             tree_type="troubleshooting",
             tree_structure=_make_tree_structure(2),
+            account_id=_PLATFORM_ACCOUNT_ID,
             is_gallery_featured=True,
             is_active=True,
             usage_count=5,