feat(download): expose aria2 disk write failure root cause at INFO level

Promote aria2 stderr lines that indicate disk write failures (e.g. the
'cause: No space left on device' line following 'Write disk cache flush
failure') from DEBUG to INFO so the root cause is visible in default logs,
including Windows-specific phrases (file locked by another process, sharing
violation). The same line is rate-limited to one INFO report per 60s window
and the report map is pruned on insert so repeated failures cannot spam the
log or grow memory. All other stderr output stays at DEBUG.

py/services/aria2_downloader.py

@@ -11,6 +11,7 @@ import os
 import secrets
 import shutil
 import socket
+import time
 from dataclasses import dataclass
 from datetime import datetime
 from pathlib import Path
@@ -28,6 +29,35 @@ logger = logging.getLogger(__name__)
 # is lost (daemon restart / RPC outage) before failing the download.
 MAX_TRANSFER_RECOVERY_ATTEMPTS = 2
 
+# stderr lines matching these markers indicate a disk write failure inside
+# aria2 (piece cache flush or raw file write).  They are promoted to INFO so
+# the root cause (disk full, permission denied, file locked by another
+# process, ...) is visible in the default logs; all other stderr output stays
+# at DEBUG to avoid noise.
+_DISK_WRITE_ERROR_MARKERS = (
+    # aria2 wrapper messages (write disk cache flush path)
+    "write disk cache flush failure",
+    "error when trying to flush write cache",
+    "failed to write into the file",
+    "failed to open the file",
+    "failed to seek the file",
+    # underlying root-cause phrases reported via "cause: ..." (POSIX + Windows)
+    "no space left on device",
+    "not enough space on the disk",
+    "input/output error",
+    "permission denied",
+    "access is denied",
+    "disk quota exceeded",
+    "used by another process",
+    "sharing violation",
+)
+
+# Minimum interval between INFO-level reports of the same stderr line so a
+# repeated failure (e.g. aria2 retrying against a full disk) does not spam
+# the log.
+STDERR_ERROR_REPORT_INTERVAL = 60.0
+
+
 def _try_certifi_ca_path() -> str | None:
     """Return the certifi CA bundle path if available, else None."""
     try:
@@ -94,6 +124,7 @@ class Aria2Downloader:
         self._poll_interval = 0.5
         self._state_store = Aria2TransferStateStore()
         self._stderr_reader_task: Optional[asyncio.Task[Any]] = None
+        self._stderr_error_report: Dict[str, float] = {}
 
     @property
     def is_running(self) -> bool:
@@ -447,16 +478,51 @@ class Aria2Downloader:
         blocks, which freezes the entire ``aria2c`` process — including its
         RPC handler.  This background task reads lines from stderr as they
         arrive and forwards them to Python's logger.
+
+        Lines that indicate a disk write failure (e.g. the "cause: No space
+        left on device" line that follows "Write disk cache flush failure")
+        are promoted to INFO so the root cause is visible without enabling
+        debug logging; every other line stays at DEBUG to avoid noise.
         """
         try:
             assert self._process is not None and self._process.stderr is not None
             async for line in self._process.stderr:
                 text = line.decode("utf-8", errors="replace").rstrip()
                 if text:
-                    logger.debug("aria2 stderr: %s", text)
+                    if self._is_disk_write_error(text):
+                        self._report_stderr_error(text)
+                    else:
+                        logger.debug("aria2 stderr: %s", text)
         except Exception:
             pass
 
+    @staticmethod
+    def _is_disk_write_error(text: str) -> bool:
+        lowered = text.lower()
+        return any(marker in lowered for marker in _DISK_WRITE_ERROR_MARKERS)
+
+    def _report_stderr_error(self, text: str) -> None:
+        """INFO-log a disk write failure line, rate-limited per line text.
+
+        aria2 re-emits the same error chain on every poll/retry while the
+        underlying condition persists; only the first occurrence within
+        ``STDERR_ERROR_REPORT_INTERVAL`` seconds is promoted to INFO.
+        """
+        now = time.monotonic()
+        last = self._stderr_error_report.get(text)
+        if last is not None and now - last < STDERR_ERROR_REPORT_INTERVAL:
+            logger.debug("aria2 stderr (repeated disk write error): %s", text)
+            return
+        # Drop entries older than the window so the map stays bounded even
+        # during a long disk-full episode (piece indexes change per line).
+        self._stderr_error_report = {
+            line: timestamp
+            for line, timestamp in self._stderr_error_report.items()
+            if now - timestamp < STDERR_ERROR_REPORT_INTERVAL
+        }
+        self._stderr_error_report[text] = now
+        logger.info("aria2 disk write failure: %s", text)
+
     async def _dispatch_progress(self, callback, snapshot: DownloadProgress) -> None:
         try:
             result = callback(snapshot, snapshot)