TODO: Clean up test seed Block objects with null anchor_ids

Test seed helpers in tests/test_blocks_api.py and tests/test_compiled_page_api.py still create Block objects with anchor_id=None. This contradicts the invariant established by PR #120 (all blocks must have anchor_ids). Works because SQLite treats NULLs as distinct for unique constraints, but is inconsistent.

Update all test seed helpers to provide anchor_id="{block_type}-{position}" for non-heading blocks, matching the parser and API behavior.

Test files only — no production code changes.