fix(api): return postgres query details

[deodad]

Summary

Return useful PostgreSQL query errors from /query instead of db error.

Motivation

PostgreSQL exposes the real failure through tokio_postgres::DbError, but the API was formatting the outer error after anyhow flattened it to db error. That made bad SQL hard to debug from API responses.

Changes

• Extract DbError fields in src/service/mod.rs before formatting query failures.
• Return sanitized message, detail, hint, and position when Postgres provides them.
• Add an API regression test in tests/api_live_test.rs for a missing column query.

Testing

• cargo fmt --check
• cargo check
• cargo test test_sanitize
• DATABASE_URL=postgresql://tidx:tidx@localhost:5433/tidx cargo test test_query_returns_postgres_error_message