perf(csharp): remove duplicate DB round-trips in update/create hot paths

[davideme]

Problem

C# request flow currently performs extra DB round-trips in hot paths:

• Update path does pre-read in controller and another read in repository.
• Create/Update paths call ReloadAsync() after SaveChangesAsync() to fetch refreshed values.

These patterns add latency and reduce throughput under load.

Goal

Reduce unnecessary round-trips in write paths while preserving API semantics and not-found behavior.

Scope

• Refactor controller/repository interaction for update/delete/create paths.
• Keep response contract unchanged.

Proposed changes

• Remove controller pre-read and let repository/service own not-found detection via affected rows or single read path.
• Eliminate unconditional ReloadAsync() on hot path where not strictly required.
• Review retry behavior for benchmark mode (EnableRetryOnFailure) to avoid artificial latency overhead in controlled tests.

Acceptance criteria

• Update path avoids duplicated pre-read + repository read pattern.
• Create/Update no longer always issue ReloadAsync() unless required for correctness.
• Not-found and status-code behavior remains unchanged.
• C# tests pass (unit/integration).
• Benchmark rerun shows improved C# DB p95/p99 and/or max stable RPS.

Notes

The objective is to keep EF Core idiomatic while minimizing per-request DB work.