Cleaner error logging: sanic loggers at INFO, 499/503 for cancelled requests, shortened preview error reasons

- Force sanic.root/error/server back to INFO from before_server_start, after
  dev mode's runtime setLevel(DEBUG) — kills the useless 'Error Page:' noise
- Silence mediapreview.pool WARNINGs (timeouts already in access log extra);
  they previously fell to logging.lastResort with no level prefix
- Handle CancelledError with 499 (client disconnect / RequestCancelled) or
  503 (server shutdown) instead of Sanic's default 500 error page
- 422 preview failures now log 'backend: reason' in the access log, with
  upstream error text shortened (first line, no [Errno] prefix, cut at ': ');
  dev mode prints the full original error to console

cista/preview.py

@@ -6,6 +6,7 @@ Sanic with auth, etag negotiation and the in-memory response cache.
 """
 
 import asyncio
+import re
 import urllib.parse
 from pathlib import PurePosixPath
 from urllib.parse import unquote
@@ -38,6 +39,21 @@ bp = Blueprint("preview", url_prefix="/preview")
 _preview_cache = PreviewCache(capacity=500)
 
 
+def _shorten_error(detail: str) -> str:
+    """Shorten an upstream backend error for the single-line access log.
+
+    Backend errors arrive verbatim from ffmpeg/pyvips/pymupdf and often carry
+    an '[Errno N]' prefix, the input file path, and multi-line library noise —
+    all redundant with the URL already in the log line. Keep the first line,
+    drop the bracket prefix, and cut at the first ': ' separator.
+    """
+    lines = detail.splitlines()
+    if not lines:
+        return ""
+    first_line = re.sub(r"^\[[^\]]*\]\s*", "", lines[0].strip())
+    return first_line.split(": ", 1)[0]
+
+
 @bp.on_request
 async def verify_preview(request):
     """Verify access to preview routes."""
@@ -125,8 +141,16 @@ async def preview(req, path):
             if captured:
                 detail = captured.splitlines()[0]
         # The worker already logged the failure (with traceback where the
-        # error occurred) — annotate the access log instead of re-logging.
-        req.ctx.log_extra = e.backend or detail
+        # error occurred) — annotate the access log instead of re-logging,
+        # with a shortened reason. In dev mode, print the full error too.
+        backend = e.backend or _expected_preview_backend(filepath)
+        if req.app.debug:
+            full = detail
+            if e.stderr and e.stderr.strip() not in detail:
+                full = f"{detail}\n{e.stderr.strip()}"
+            logger.warning("[%s] preview failed: %s", backend, full.strip())
+        short = _shorten_error(detail)
+        req.ctx.log_extra = f"{backend}: {short}" if short else backend
         return empty(422)
     except asyncio.CancelledError:
         # Server shutdown or client disconnect: the connection is being torn