Link previews: Open Graph tags + logo for shared URLs (#35)

Sharing a Thermograph URL (Discord, Slack, iMessage…) now unfurls into a
card with the site name, page title, description, accent color, and logo.

- All five pages get description/theme-color/og:*/twitter:card meta and a
  favicon. og:url/og:image need absolute URLs and crawlers don't run JS, so
  pages carry an __ORIGIN__ placeholder the server fills in per request from
  X-Forwarded-Proto (Caddy) / scheme + the Host header + base path — correct
  on both the LAN dev server and the prod domain without hardcoding either.
- New logo assets: the header's ▚ mark as an app icon (accent quadrants on
  the dark surface) — logo.png (512x512, the og:image) and logo.svg (favicon).
- Page routes render the substitution with a weak ETag + 304 revalidation
  (replacing plain FileResponse) and now answer HEAD, which preview crawlers
  probe with (previously 404).

app.py

@@ -10,7 +10,7 @@ import time
 import pandas as pd
 from fastapi import APIRouter, FastAPI, HTTPException, Query, Request, Response
 from fastapi.middleware.gzip import GZipMiddleware
-from fastapi.responses import FileResponse, RedirectResponse
+from fastapi.responses import RedirectResponse
 from fastapi.staticfiles import StaticFiles
 
 import audit
@@ -784,18 +784,37 @@ app.include_router(v2, prefix=f"{BASE}/api/v2")
 
 # --- static frontend (served under BASE) -----------------------------------
 def _page(name):
-    return FileResponse(os.path.join(FRONTEND_DIR, name))
+    """Route handler for one HTML page. Serves the file with its __ORIGIN__
+    placeholders (the link-preview/Open Graph tags) filled in as the request's
+    scheme://host + BASE — preview crawlers (Discord, Slack, …) need absolute
+    URLs, and the host differs between LAN and prod. The scheme comes from
+    X-Forwarded-Proto when a reverse proxy (Caddy) fronts the plain-HTTP
+    uvicorn; the proxy passes the original Host header through untouched."""
+    path = os.path.join(FRONTEND_DIR, name)
+
+    def route(request: Request):
+        with open(path, encoding="utf-8") as f:
+            html = f.read()
+        proto = request.headers.get("x-forwarded-proto") or request.url.scheme
+        host = request.headers.get("host") or request.url.netloc
+        html = html.replace("__ORIGIN__", f"{proto}://{host}{BASE}")
+        etag = f'W/"{hashlib.sha1(html.encode()).hexdigest()[:20]}"'
+        if _not_modified(request, etag):
+            return Response(status_code=304, headers={"ETag": etag})
+        return Response(html, media_type="text/html", headers={"ETag": etag})
+    return route
 
 
 # The un-slashed base redirects to BASE/ so the frontend's relative asset URLs
 # resolve correctly. The app stays scoped to BASE and never claims "/", leaving
 # the domain root free for another app (e.g. a portfolio) to own via the proxy.
-app.add_api_route(BASE, lambda: RedirectResponse(url=f"{BASE}/"), methods=["GET"], include_in_schema=False)
-app.add_api_route(f"{BASE}/", lambda: _page("index.html"), methods=["GET"], include_in_schema=False)
-app.add_api_route(f"{BASE}/calendar", lambda: _page("calendar.html"), methods=["GET"], include_in_schema=False)
-app.add_api_route(f"{BASE}/day", lambda: _page("day.html"), methods=["GET"], include_in_schema=False)
-app.add_api_route(f"{BASE}/compare", lambda: _page("compare.html"), methods=["GET"], include_in_schema=False)
-app.add_api_route(f"{BASE}/legend", lambda: _page("legend.html"), methods=["GET"], include_in_schema=False)
+# Pages also answer HEAD — link-preview crawlers probe with it before fetching.
+app.add_api_route(BASE, lambda: RedirectResponse(url=f"{BASE}/"), methods=["GET", "HEAD"], include_in_schema=False)
+app.add_api_route(f"{BASE}/", _page("index.html"), methods=["GET", "HEAD"], include_in_schema=False)
+app.add_api_route(f"{BASE}/calendar", _page("calendar.html"), methods=["GET", "HEAD"], include_in_schema=False)
+app.add_api_route(f"{BASE}/day", _page("day.html"), methods=["GET", "HEAD"], include_in_schema=False)
+app.add_api_route(f"{BASE}/compare", _page("compare.html"), methods=["GET", "HEAD"], include_in_schema=False)
+app.add_api_route(f"{BASE}/legend", _page("legend.html"), methods=["GET", "HEAD"], include_in_schema=False)
 
 # Everything else under BASE (app.js, style.css, nav.js, …) is a static asset.
 # Registered last so the explicit page routes above win.