feat: PWA update strategy with hashed assets and cache control

Move frontend sources to frontend/, build content-hashed assets at deploy,
network-first SW for shell/JS, immutable cache for /assets/, version.json
update banner, and Cache-Control middleware. Docs in docs/PWA-STRATEGY.md.

Co-authored-by: Cursor <cursoragent@cursor.com>

.gitignore

@@ -1,5 +1,7 @@
 .env
 data/
+public/assets/*
+!public/assets/.gitkeep
 __pycache__/
 *.py[cod]
 *.egg-info/
@@ -7,3 +9,5 @@ __pycache__/
 venv/
 .DS_Store
 *.egg-info/
+public/assets/*
+!public/assets/.gitkeep