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_db_schema.py

@@ -3,7 +3,7 @@
 `init_db` is called unconditionally on startup, so it must be idempotent
 and create every table/trigger the rest of the app expects.
 """
-from db import get_db, return_db, init_db, get_setting, set_setting, get_site_name
+from tinyweb.db import get_db, return_db, init_db, get_setting, set_setting, get_site_name
 
 
 EXPECTED_TABLES = {