Fix critical admin deletion and valuation regressions

[cursor]

Bug and impact

• Admin user deletion could cascade through executions -> reports -> report_views, deleting shared analysis reports and other users' view history when deleting a content creator. The system account was also deletable, risking loss of cron/sync-generated platform reports.
• ETF/index detection treated equity names containing generic words like Trust or Fund as ETF/index instruments, causing REITs and similar equities to skip stock DCF/comps valuation and silently show the wrong valuation methodology.
• Valuation structured output capped valuation_key_assumptions at 5 even though the deterministic tool can emit more, causing structured valuation metadata to be dropped on validation failure.

Root cause

• admin_service.delete_user deleted User rows directly without preserving owned executions/reports or protecting platform/admin accounts.
• _is_index_or_etf used broad substring matching across names and metadata instead of respecting explicit instrument types.
• The Pydantic output schema was stricter than the tool output and prompt contract.

Fix

• Reassign deleted users' owned executions to the system account before deleting the personal account, and reject deletion of admin/system accounts.
• Narrow ETF/index detection to explicit instrument types first, treat explicit equities as equities, and remove generic name-only Trust/Fund matches.
• Allow the structured valuation output to carry the tool's full assumption list.
• Update the admin deletion dialog copy so it no longer claims analysis runs are deleted.

Validation

• python3 -m unittest backend.tests.test_admin_service backend.tests.test_valuation_summary