Cache-bust style.css so a deploy doesn't leave visitors on a stale copy

Browsers apply much more aggressive heuristic caching to a linked
stylesheet than to the dynamically-rendered HTML that references it, so a
CSS-only deploy could silently leave an already-open browser serving the
old styles until a hard refresh -- exactly what made today's dashboard
polish changes look broken while testing, until forcing a fresh fetch.
Appends a content-hash query param computed once at startup.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

app/web/routes.py

@@ -1,5 +1,8 @@
 from __future__ import annotations
 
+import hashlib
+from pathlib import Path
+
 from fastapi import APIRouter, Request
 from fastapi.templating import Jinja2Templates
 
@@ -9,6 +12,23 @@ router = APIRouter()
 templates = Jinja2Templates(directory="app/web/templates")
 
 
+def _asset_version(path: str) -> str:
+    """A short hash of a static asset's content, used to cache-bust it in
+    templates -- browsers apply much more aggressive heuristic caching to
+    linked CSS/JS than to the dynamically-rendered HTML that references it,
+    so without this, a deploy that only changes style.css can leave a
+    visitor's browser silently serving the old stylesheet until they
+    hard-refresh. Computed once at startup since the file is baked into the
+    image and won't change while this process is running."""
+    try:
+        return hashlib.md5(Path(path).read_bytes()).hexdigest()[:8]
+    except FileNotFoundError:
+        return "0"
+
+
+templates.env.globals["static_version"] = _asset_version("app/static/css/style.css")
+
+
 @router.get("/")
 def dashboard(request: Request):
     return templates.TemplateResponse(request, "dashboard.html", {})

app/web/templates/base.html

@@ -5,7 +5,7 @@
   <meta name="viewport" content="width=device-width, initial-scale=1.0">
   <title>Power Compare</title>
   <script src="https://cdn.jsdelivr.net/npm/chart.js@4"></script>
-  <link rel="stylesheet" href="/static/css/style.css">
+  <link rel="stylesheet" href="/static/css/style.css?v={{ static_version }}">
 </head>
 <body>
   <nav class="topnav">