[amodelandme]

• Map Flag.Version (uint) to the Postgres xmin system column with IsRowVersion(). EF includes it in the WHERE clause on every UPDATE/DELETE, so conflicting writes raise
  DbUpdateConcurrencyException instead of silently last-write-winning.
• Translate DbUpdateConcurrencyException at the repository edge into a new FlagConcurrencyException (HTTP 409), mirroring the existing 23505 → DuplicateFlagNameException pattern. Application and API
  layers stay free of EF Core types.
• Integration test proves the token fires: two scopes load the same flag, scope A saves, scope B's save throws.

Why xmin over byte[] Version

byte[] Version + IsRowVersion() is the canonical EF Core row-version pattern but only works on SQL Server. On Npgsql it produces a bytea column the database never auto-updates — the concurrency check
becomes a silent no-op. uint Version lets the Npgsql provider map to xmin, which Postgres maintains on every row change for free. The generated migration emits no real DDL because xmin is a system column
already present on every Postgres row.

Follow-ups (not in this PR)

• API-level test issuing concurrent PUT /api/flags/... requests and asserting a 409 ProblemDetails response.
• Revisit if a detach/reattach code path is added (rename property to xmin to be safe).

Test plan

• dotnet build Banderas.sln -p:TreatWarningsAsErrors=true — 0 warnings, 0 errors
• dotnet test Banderas.sln --filter "FullyQualifiedName~FlagConcurrencyTokenTests" — passes
• Full suite: dotnet test Banderas.sln
• dotnet ef database update applies cleanly on a fresh Postgres
• Manual: load a flag, mutate via two clients, confirm second receives 409