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_pages.py

@@ -4,8 +4,8 @@ The bulk-delete confirmation flow is a data-loss guard added in commit
 8dffd8c — a stray POST without `confirmed=1` must render the confirmation
 page instead of actually deleting.
 """
-from db import get_db, return_db
-from handlers import (
+from tinyweb.db import get_db, return_db
+from tinyweb.handlers import (
     handle_bulk_action,
     handle_edit_form,
     handle_edit_submit,