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_handlers_search.py

@@ -1,5 +1,5 @@
 """Tests for `handle_search` — the home page + primary user flow."""
-from handlers import handle_search
+from tinyweb.handlers import handle_search
 
 
 def test_empty_index_empty_query_shows_welcome(temp_db, csrf_session):