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_sharing_logic.py

@@ -6,7 +6,7 @@ hiding pages the user meant to share — both are worth a regression net.
 """
 import pytest
 
-from handlers import _page_is_shared
+from tinyweb.handlers import _page_is_shared
 
 
 @pytest.mark.parametrize("mode", ["exclude_private", "require_public"])