Picture: download URLs once at save-time, not on every poll

Previously check() re-fetched the configured image URL over HTTP on
every poll (every 30 min by default). If the remote image ever came
back even slightly byte-different between fetches -- common with
dynamically-generated images -- that forced a real e-ink flash for no
visible reason, since this panel has no partial refresh.

Now the URL is downloaded exactly once, server-side in the settings
route, and saved into web/static/uploads/current.<ext> the same way a
direct upload already was -- clearing any previous upload/download
first so switching sources doesn't accumulate orphaned files.
check() just reads that local file regardless of whether it came from
an upload or a URL, with zero network calls. Also added a browser
User-Agent to the download request -- confirmed live that Wikimedia
(plausibly a common source people would link to) 403s the default
requests/python User-Agent as bot traffic, same class of issue as the
EFA transit endpoints handled earlier.

Verified end-to-end against real URLs: download+cache works, a second
URL replaces rather than accumulates, and a settings-only save (no
new URL entered) no longer wipes the cached picture.

web/app.py

@@ -1,5 +1,7 @@
 """Flask app: status/preview page + one settings form per screen + rotation settings."""
 
+from io import BytesIO
+
 import requests
 from flask import Flask, redirect, render_template, request, url_for
 from PIL import Image
@@ -8,6 +10,11 @@ from werkzeug.utils import secure_filename
 from screens.picture import UPLOAD_DIR
 
 ALLOWED_IMAGE_EXTENSIONS = {"png", "jpg", "jpeg", "gif", "bmp", "webp"}
+PIL_FORMAT_TO_EXT = {"JPEG": "jpg", "PNG": "png", "GIF": "gif", "BMP": "bmp", "WEBP": "webp"}
+PICTURE_URL_TIMEOUT = 15
+PICTURE_URL_HEADERS = {
+    "User-Agent": "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/125.0.0.0 Safari/537.36",
+}
 
 
 def create_app(config, screens: dict, scheduler) -> Flask:
@@ -257,9 +264,48 @@ def create_app(config, screens: dict, scheduler) -> Flask:
                         scheduler.invalidate_screen("picture")
                         return redirect(url_for("settings_picture", saved=1))
             elif url:
-                config.update_screen_settings("picture", {"mode": "url", "image_url": url, **common_updates})
-                scheduler.invalidate_screen("picture")
-                return redirect(url_for("settings_picture", saved=1))
+                # Downloaded once, here, rather than re-fetched by the
+                # screen's own check() on every poll -- otherwise a remote
+                # image that comes back even slightly byte-different between
+                # polls (common with dynamically-generated images) would
+                # force a real e-ink flash every poll_interval for no
+                # visible reason.
+                try:
+                    # Some image hosts (Wikimedia confirmed live -- 403s with
+                    # "set a user-agent") reject the default requests/python
+                    # User-Agent as bot traffic.
+                    response = requests.get(url, timeout=PICTURE_URL_TIMEOUT, headers=PICTURE_URL_HEADERS)
+                    response.raise_for_status()
+                    content = response.content
+                    fmt = Image.open(BytesIO(content)).format
+                except (requests.exceptions.RequestException, OSError):
+                    fmt = None
+                ext = PIL_FORMAT_TO_EXT.get(fmt or "")
+                if not ext:
+                    error = "Could not download a valid image from that URL -- try a different link, or upload a file instead."
+                else:
+                    UPLOAD_DIR.mkdir(parents=True, exist_ok=True)
+                    # Only one picture is ever active -- clear any previous
+                    # upload/download first so a different extension doesn't
+                    # leave an orphaned file behind.
+                    for old in UPLOAD_DIR.glob("current.*"):
+                        old.unlink()
+                    dest = UPLOAD_DIR / f"current.{ext}"
+                    dest.write_bytes(content)
+                    try:
+                        Image.open(dest).verify()
+                    except Exception:
+                        dest.unlink(missing_ok=True)
+                        error = "That URL doesn't look like a valid image."
+                    else:
+                        config.update_screen_settings("picture", {
+                            "mode": "url",
+                            "image_url": url,
+                            "uploaded_filename": dest.name,
+                            **common_updates,
+                        })
+                        scheduler.invalidate_screen("picture")
+                        return redirect(url_for("settings_picture", saved=1))
             else:
                 # Neither a new upload nor a URL change -- just update fit/boost/grayscale.
                 config.update_screen_settings("picture", common_updates)

web/templates/settings_picture.html

@@ -6,17 +6,15 @@
 <div class="toast error">{{ error }}</div>
 {% endif %}
 
-{% if settings.mode == "upload" and settings.uploaded_filename %}
+{% if settings.uploaded_filename %}
 <div class="card">
   <div class="section-label">Current picture</div>
   <div class="preview-wrap">
     <img class="preview" src="{{ url_for('static', filename='uploads/' + settings.uploaded_filename) }}" alt="current picture">
   </div>
-</div>
-{% elif settings.mode == "url" and settings.image_url %}
-<div class="card">
-  <div class="section-label">Current picture</div>
-  <p class="hint">Fetched from a URL: {{ settings.image_url }}</p>
+  {% if settings.mode == "url" and settings.image_url %}
+  <p class="hint">Downloaded from: {{ settings.image_url }}</p>
+  {% endif %}
 </div>
 {% endif %}
 
@@ -35,7 +33,7 @@
     <div class="field">
       <label for="image_url">Image URL</label>
       <input type="text" id="image_url" name="image_url" placeholder="https://example.com/photo.jpg">
-      <p class="hint">Re-fetched every 30 minutes. Leave both this and the upload above alone to just change the options below.</p>
+      <p class="hint">Downloaded once and cached -- won't keep re-fetching, so this can't cause the panel to flash on its own. Enter a new URL any time to replace it. Leave both this and the upload above alone to just change the options below.</p>
     </div>
   </div>