Picture: surface the real reason a URL download fails

Couldn't reproduce the reported URL failing -- downloads fine from
here (200, valid 2.5MB PNG, verified through the actual running app
too) -- but found the error handling was swallowing the real
exception either way: network failure and "not a valid image" both
landed on the same generic message, and nothing was logged, so there
was no way to tell which one it actually was, from the Pi or from
here.

Now: timeouts, connection/HTTP errors, and decode failures each get
their own specific error message, and failures get logged via the
standard logger (visible in journalctl -u paperwall.service). Also
bumped the download timeout 15s -> 30s -- it's a one-time save-time
download now, not a recurring poll, so there's no real cost to being
more patient with a slow host or a large image.

Verified against the real app: the exact reported URL, a non-image
URL, and an unresolvable domain each now produce distinct, logged
error messages instead of one generic one.

web/app.py

@@ -1,5 +1,6 @@
 """Flask app: status/preview page + one settings form per screen + rotation settings."""
 
+import logging
 from io import BytesIO
 
 import requests
@@ -9,9 +10,11 @@ from werkzeug.utils import secure_filename
 
 from screens.picture import UPLOAD_DIR
 
+logger = logging.getLogger(__name__)
+
 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_TIMEOUT = 30  # a one-time download at save-time, not a recurring poll -- fine to be generous
 MAX_SCHEDULE_RULES = 4
 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",
@@ -244,12 +247,25 @@ def create_app(config, screens: dict, scheduler) -> Flask:
                     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
+                except requests.exceptions.Timeout:
+                    content = fmt = None
+                    error = f"Timed out downloading that URL after {PICTURE_URL_TIMEOUT}s -- the host might be slow or blocking this server."
+                except requests.exceptions.RequestException as exc:
+                    content = fmt = None
+                    logger.warning("Picture URL download failed for %s: %s", url, exc)
+                    error = f"Could not download that URL -- {exc}"
+                else:
+                    try:
+                        fmt = Image.open(BytesIO(content)).format
+                    except OSError as exc:
+                        logger.warning("Picture URL %s downloaded but didn't decode as an image: %s", url, exc)
+                        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."
+                    if not error:
+                        error = "That URL doesn't look like a valid image -- 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