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_gateway_limits.py

@@ -8,8 +8,8 @@ import io
 
 import pytest
 
-import app as app_module
-from gateway import GatewayHandler, MAX_BODY_SIZE
+from tinyweb import app as app_module
+from tinyweb.gateway import GatewayHandler, MAX_BODY_SIZE
 
 
 class FakeHeaders:
@@ -72,7 +72,7 @@ def test_post_at_size_cap_accepted():
         rfile=io.BytesIO(b""),
     )
     # Stub out local_dispatch so _forward doesn't try the network path.
-    from gateway import GatewayState
+    from tinyweb.gateway import GatewayState
     original = GatewayState.local_dispatch
     GatewayState.local_dispatch = lambda data: {
         "status": 404, "content_type": "text/plain", "body": "nope",