src layout: move core code into src/tinyweb/ package

- Moved app.py, db.py, gateway.py, templates.py, embeddings.py,
  rns_client.py, and handlers/ into src/tinyweb/
- Created root app.py shim (adds src/ to sys.path, imports main)
- Created pyproject.toml with setuptools config (where = ["src"])
- Added src/tinyweb/__init__.py
- Updated all internal imports to use tinyweb. prefix (73 occurrences)
- Removed sys.path.insert hack from conftest.py
- Updated Dockerfile: pip install -e /app before running
- Updated gateway.py usage message: python -m tinyweb.gateway
- Updated README.md gateway usage instructions

pyproject.toml

@@ -0,0 +1,12 @@
+[project]
+name = "tinyweb"
+version = "0.1.0"
+description = "Personal decentralized search engine"
+requires-python = ">=3.10"
+
+[tool.setuptools.packages.find]
+where = ["src"]
+
+[build-system]
+requires = ["setuptools"]
+build-backend = "setuptools.backends._legacy:_Backend"