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

conftest.py

@@ -5,15 +5,11 @@ per-test tempfile, `seeded_db` layers sample rows on top, and `csrf_session`
 primes the thread-local CSRF token that handlers read.
 """
 import socket
-import sys
-from pathlib import Path
 
 import pytest
 
-sys.path.insert(0, str(Path(__file__).parent))
-
-import db as db_module
-import handlers as handlers_module
+import tinyweb.db as db_module
+import tinyweb.handlers as handlers_module
 
 
 @pytest.fixture