move server code into server/, update docs

- app.py + upstream.py -> server/ (relative import, __init__.py)
- static/ resolved relative to repo root, works from any CWD
- watch history migrated to server/data/history.json
- README.md: quick start, features, layout, dev notes
- SPEC.md: marked implemented, updated endpoints/run/deviations
- ignore *.tar.gz (local curl-impersonate download at repo root)

.gitignore

@@ -1,5 +1,6 @@
 .venv/
 __pycache__/
 *.pyc
-data/
+*.tar.gz
+server/data/
 server.log