Fix infinite loop in find_unprocessed: check sha256 content (not just path), record dup paths; Optional[int] for max_batches

dashboard/app.py

@@ -246,6 +246,12 @@ def upload_page(request: Request):
     return templates.TemplateResponse(request, "upload.html", {})
 
 
+@app.get("/howto", response_class=HTMLResponse)
+def howto(request: Request):
+    """How-to documentation page."""
+    return templates.TemplateResponse(request, "howto.html", {})
+
+
 @app.post("/upload")
 async def upload(request: Request):
     """Receive uploaded files → save to /mnt/data/takeout/incoming/."""