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

tests/test_link_extraction.py

@@ -7,7 +7,7 @@ skip Wikipedia special pages, resolve relatives via urljoin.
 from unittest.mock import patch
 
 from conftest import patch_dns_ok
-import db as db_module
+import tinyweb.db as db_module
 
 
 class FakeResponse: