Feat: Gate API docs endpoints behind API_DOCS_ENABLED env var

When API_DOCS_ENABLED=false, FastAPI registers no routes for /docs,
/redoc, or /openapi.json, returning 404 for all three. Default is true
for backwards compatibility. Invalid values fall back to true (FR-007).

Fix: Remove tests/ and alembic/ from api/.dockerignore so the test
Dockerfile (which uses COPY . .) includes the test suite; Dockerfile.prod
is unaffected as it only copies app/ explicitly.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

api/app/main.py

@@ -33,7 +33,16 @@ async def lifespan(application: FastAPI):
     await engine.dispose()
 
 
-app = FastAPI(title="Reactbin API", version="1.0.0", lifespan=lifespan)
+_settings = get_settings()
+
+app = FastAPI(
+    title="Reactbin API",
+    version="1.0.0",
+    lifespan=lifespan,
+    docs_url="/docs" if _settings.api_docs_enabled else None,
+    redoc_url="/redoc" if _settings.api_docs_enabled else None,
+    openapi_url="/openapi.json" if _settings.api_docs_enabled else None,
+)
 
 # Defaults so app.state is populated even when lifespan doesn't run (e.g. tests)
 app.state.login_rate_limiter = LoginRateLimiter()