fix(csv): a blank MemoryDB engine column still slips past replay dedupe #2232

Follow-up from the independent review of #2230 (CSV replay dedupe).

Problem

With #2230, csvDetails builds CacheDetails{Engine: <CSV engine column>} for MemoryDB. If the Engine column is blank (a hand-written or stripped CSV), the MemoryDB rec's dedupe key engine is "" while the existing commitment's key engine is "redis". The duplicate check therefore doesn't match, and an --input-csv replay buys the reservation again. The review reproduced this with the real CSV loader and DuplicateChecker: a blank-engine memorydb row lands in the would-buy list.

This is low likelihood: the CSV writer always emits redis for MemoryDB. But this is a real-money path, so it should fail closed.

Suggested fix

Either refuse a MemoryDB CSV row with an empty engine (preferred: fail loud), or default it to redis to match the main recommendation path (CacheDetails{Engine:"redis"}). Then audit the other services' CSV-detail columns for the same blank-value fail-open, and add a replay test for the blank-engine case.