fix(backend): gate Swagger + the OpenAPI doc behind IsDevelopment (RB-15)

BIO-015: app.UseSwagger()/app.UseSwaggerUI() ran unconditionally, so
the full OpenAPI document (every route + request/response shape) and
SwaggerUI's interactive "Try it out" were reachable in every
environment, including a real deployment.

Both now run only inside `if (app.Environment.IsDevelopment())`.
AddSwaggerGen/AddEndpointsApiExplorer stay unconditional — DI
registration only, no HTTP surface by itself.

RB-09 already made a non-Development environment throw at startup,
which broke `npm run gen:api` until that script pinned
ASPNETCORE_ENVIRONMENT=Development for its one CLI invocation. This
change sits in the same pipeline, so it was verified rather than
assumed: `dotnet swagger tofile` resolves ISwaggerProvider straight
out of DI and never sends an HTTP request through this middleware, so
gating it can't affect that tool by construction. Ran the real
`npm run gen:api` to confirm — exit 0, regenerated files byte-identical
to what's committed.

New tests exercise the gate on a third ("Staging") environment name,
not Production — Production already can't boot at all post-RB-09, so
a Production-environment test would only re-prove that unrelated
startup throw, not this gate.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

backend/src/BigRegister.Api/Program.cs

@@ -142,8 +142,21 @@ app.Use(async (ctx, next) =>
   await next(ctx);
 });
 
-app.UseSwagger();
-app.UseSwaggerUI();
+// RB-15/BIO-015: the OpenAPI document + its UI are a genuine attack-surface reduction to
+// gate — they enumerate every route, request/response shape and (via SwaggerUI's "Try it
+// out") let a caller fire requests straight from the browser. Development-only, like the
+// dev-role/scenario-toggle hatches this POC already keeps out of production builds
+// (docker-compose.prod.yml runs Production; only docker-compose.yml's dev image runs
+// Development). `dotnet swagger tofile` (npm run gen:api) is unaffected: Swashbuckle's CLI
+// resolves ISwaggerProvider straight out of the DI container to build swagger.json — it
+// never sends an HTTP request through this pipeline, so it never touches this middleware at
+// all, gated or not. Verified empirically (see rb-15.md) rather than assumed, per RB-09's
+// note that this exact file has already broken that tool once.
+if (app.Environment.IsDevelopment())
+{
+  app.UseSwagger();
+  app.UseSwaggerUI();
+}
 app.UseCors(SpaCors);
 
 // Liveness/readiness for orchestrators (k8s probes, load balancers). No data, no PII.

backend/tests/BigRegister.Tests/SwaggerGateTests.cs

@@ -0,0 +1,48 @@
+using System.Net;
+using BigRegister.Domain.Authorization;
+using Microsoft.AspNetCore.Hosting;
+using Microsoft.AspNetCore.TestHost;
+using Microsoft.Extensions.DependencyInjection;
+
+namespace BigRegister.Tests;
+
+/// RB-15/BIO-015: `app.UseSwagger()`/`app.UseSwaggerUI()` used to run unconditionally — the
+/// OpenAPI document (every route + request/response shape) and SwaggerUI's "Try it out" were
+/// reachable in every environment, including a real deployment. Both are now gated behind
+/// `app.Environment.IsDevelopment()`.
+public class SwaggerGateTests(TestWebApplicationFactory factory) : IClassFixture<TestWebApplicationFactory>
+{
+  [Fact]
+  public async Task Swagger_document_is_served_in_development()
+  {
+    // The default test environment (WebApplicationFactory<T> defaults to "Development" when
+    // nothing overrides it — same fact RB-09's implementation note relies on) — this is the
+    // regression guard that the gate didn't also break the documented `npm run gen:api` /
+    // local-dev-Swagger-UI experience.
+    var res = await factory.CreateClient().GetAsync("/swagger/v1/swagger.json");
+    Assert.Equal(HttpStatusCode.OK, res.StatusCode);
+  }
+
+  /// Production cannot boot at all today (RB-09: no real IIdentityProvider exists yet), which
+  /// is a *stronger* guarantee than "no Swagger in Production" — but it also means a plain
+  /// `UseEnvironment("Production")` host never reaches this middleware to prove the gate
+  /// itself works, only that the whole app refuses to start. This uses a third environment
+  /// name (neither "Development" nor "Production") with a test-supplied `IIdentityProvider` —
+  /// the one thing Program.cs doesn't register outside those two branches — so the host
+  /// actually boots and this test exercises the real gate, not RB-09's unrelated startup throw.
+  [Fact]
+  public async Task Swagger_document_is_not_served_outside_development()
+  {
+    // Built on top of the shared `factory` fixture (via WithWebHostBuilder), not a bare `new
+    // WebApplicationFactory<Program>()` — that keeps this host on the fixture's own per-class
+    // isolated AppDb temp path (see TestWebApplicationFactory's doc comment; RB-12's
+    // implementation note records the "table already exists" collision a bare factory hits
+    // by sharing the mutable static Db.ConnectionString instead).
+    using var staging = factory.WithWebHostBuilder(builder => builder
+        .UseEnvironment("Staging")
+        .ConfigureTestServices(services => services.AddSingleton<IIdentityProvider, StubIdentityProvider>()));
+
+    var res = await staging.CreateClient().GetAsync("/swagger/v1/swagger.json");
+    Assert.Equal(HttpStatusCode.NotFound, res.StatusCode);
+  }
+}