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_admin_gallery.py

@@ -9,6 +9,7 @@ from sqlalchemy.ext.asyncio import AsyncSession
 from app.models.tree import Tree
 from app.models.script_template import ScriptTemplate, ScriptCategory
 
+_PLATFORM_ACCOUNT_ID = uuid.UUID("00000000-0000-0000-0000-000000000001")
 
 # ---------------------------------------------------------------------------
 # Helpers
@@ -22,6 +23,7 @@ async def _create_tree(db: AsyncSession, admin_user_id: str) -> Tree:
         name="Gallery Test Flow",
         tree_type="troubleshooting",
         visibility="public",
+        account_id=_PLATFORM_ACCOUNT_ID,
         is_gallery_featured=False,
         gallery_sort_order=0,
         tree_structure={
@@ -53,6 +55,7 @@ async def _create_script(db: AsyncSession, admin_user_id: str) -> ScriptTemplate
     script = ScriptTemplate(
         id=uuid.uuid4(),
         category_id=category.id,
+        account_id=_PLATFORM_ACCOUNT_ID,
         name="Gallery Test Script",
         slug=f"gallery-test-script-{uuid.uuid4().hex[:6]}",
         script_body="Write-Host 'Test'",