fix: promote httpx to production dependencies

httpx is imported unconditionally in muxplex/main.py (module-level import),
so it must be a production dependency — not dev-only. Move httpx>=0.27.0
from [project.optional-dependencies].dev to [project.dependencies] to
prevent ModuleNotFoundError on startup in non-dev deployments.

pyproject.toml

@@ -18,11 +18,11 @@ dependencies = [
     "six>=1.16.0",              # undeclared dep of python-pam (needed on macOS clean envs)
     "itsdangerous>=2.1.0",
     "python-multipart>=0.0.9",
+    "httpx>=0.27.0",
 ]
 
 [project.optional-dependencies]
 dev = [
-    "httpx>=0.27.0",
     "pytest>=8.0.0",
     "pytest-asyncio>=0.23.0",
     "beautifulsoup4>=4.12",