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

README.md

@@ -163,7 +163,7 @@ The `/export` page produces a JSON dump of your pages. It's a migration aid —
 To browse a remote TinyWeb instance without running your own index:
 
 ```bash
-python gateway.py <destination_hash>
+python -m tinyweb.gateway <destination_hash>
 ```
 
 This connects over Reticulum and serves the remote instance at `http://localhost:8080`.