chore: retire requirements.txt in favor of pyproject.toml

Addresses code quality suggestion from review: requirements.txt was a
redundant secondary source of truth. Dependencies are now exclusively
managed in pyproject.toml with proper runtime/dev separation.

- requirements.txt: replaced with a comment redirecting to pip install -e '[dev]'
- README.md: updated install command from pip install -r requirements.txt
  to pip install -e '[dev]'

requirements.txt

@@ -1,8 +1,6 @@
-fastapi>=0.115.0
-uvicorn[standard]>=0.30.0
-httpx>=0.27.0
-pytest>=8.0.0
-pytest-asyncio>=0.23.0
-aiofiles>=23.0
-beautifulsoup4>=4.12
-websockets>=11.0
+# Dependencies are now managed in pyproject.toml.
+# Install runtime + dev dependencies with:
+#
+#   pip install -e ".[dev]"
+#
+# This file is kept for historical reference only and is no longer authoritative.