fix: harden SQL connection lifecycle

[j6k4m8]

Summary

• serialize operations that share a SQLAlchemy connection
• add context-manager ownership for SQL backends
• make close idempotent and reject mutations after closure
• roll back pending transactions before shutdown
• close the connection and dispose the engine pool
• cover concurrent mutations and lifecycle cleanup

[codspeed-hq]

Merging this PR will not alter performance

✅ 8 untouched benchmarks

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_{Comparing fix/sql-lifecycle-85 (cb86c84) with master (786a8d6)}