fix: replace account_id=None with PLATFORM_ACCOUNT_ID for global content

After migration 174f442795b7 enforces NOT NULL on account_id, all
platform/global content must use the sentinel platform account instead
of NULL. Three categories of fixes:

1. trees.py: is_default trees now get PLATFORM_ACCOUNT_ID (not None)
2. admin_categories.py: global category CRUD now uses PLATFORM_ACCOUNT_ID
3. categories.py, tags.py, step_categories.py: creation endpoints coerce
   None → PLATFORM_ACCOUNT_ID; IS NULL filter queries updated to
   == PLATFORM_ACCOUNT_ID (IS NULL queries returned empty after migration
   backfilled all global rows to the platform account)

Defines PLATFORM_ACCOUNT_ID constant in app/core/service_account.py.

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

backend/app/api/endpoints/trees.py

@@ -29,6 +29,7 @@ from app.core.subscriptions import check_tree_limit, get_account_subscription, g
 from app.core.audit import log_audit
 from app.core.config import settings
 from app.core.tree_validation import can_publish_tree
+from app.core.service_account import PLATFORM_ACCOUNT_ID
 from app.core.step_sync import sync_steps_from_tree, deactivate_synced_steps_for_tree
 from app.services.rag_service import index_tree as rag_index_tree
 
@@ -470,7 +471,7 @@ async def create_tree(
         tree_structure=tree_data.tree_structure,
         intake_form=intake_form_data,
         author_id=service_account_id if is_default else current_user.id,
-        account_id=None if is_default else current_user.account_id,
+        account_id=PLATFORM_ACCOUNT_ID if is_default else current_user.account_id,
         is_public=True if is_default else tree_data.is_public,  # Default trees are always public
         is_default=is_default,
         status=tree_data.status