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>

server/settings.py

@@ -22,7 +22,7 @@ class Settings(BaseSettings):
     messages_path: str = "messages.yaml"
 
     public_dir: str = "public"
-    app_version: str = "1.0.0"
+    app_version: str = "1.1.0"
 
 
 settings = Settings()  # type: ignore[call-arg]