Init() created a new SynologyClient and logged in for every download. A
many-file download opened dozens of near-simultaneous sessions for one DSM
account; DSM invalidates older session IDs, so in-flight File Station calls
failed with error 119 "SID not found".
Add SynologyClientProvider: a thread-safe cache that authenticates once and
shares the client across downloads (concurrent first calls collapse to a
single login; later calls reuse it; a credential change re-authenticates).
Wrap every Synology call in a session-retry that drops the session and
re-authenticates once on a session error (119/106/107) so an expired SID
self-heals, reading credentials fresh from settings so edits take effect live.
Adds provider unit tests and a downloader test covering the 119 re-auth path.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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