fix: persist rerun SQL results across conversation reloads

[lindixu6-hash]

Problem

Issue #263 reproduces a persistence mismatch:

1. edit an SQL string
2. press Run
3. save the SQL
4. leave and reopen the conversation

The rerun result only replaced React state. The backend returned an ephemeral SQL_QUERY_RUN_RESULT and never updated the stored linked result, so reopening loaded the old rows.

Fix

• make the state-changing run-sql route POST
• update the existing linked run result, or create it for historical data that has none
• return the persisted result identity and timestamp to the frontend
• replace local state with the complete persisted result
• preserve the existing Secure Data marker
• reject a linked SQL result from another conversation

Verification

• focused persistence and API contract tests: 6 passed
• full non-expensive backend suite: 31 passed / 6 existing explicit skips
• database reload regression: writes old rows, updates, expires ORM state, reloads the same result ID, and observes new rows
• changed-file Black and frontend ESLint: pass
• backend compileall: pass
• frontend TypeScript + Vite production build: pass
• remote fork Backend/Frontend CI: https://github.com/lindixu6-hash/dataline/actions/runs/32064489534

No OpenAI calls, production credentials, hosted services, user data, deployment, or release workflows are used.

Closes #263.