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>