feat: add SyncManager for delta asset sync via GitHub Tree API

[raman78]

Problem

The current Downloader.download_image_list() approach re-downloads every asset that is missing from the local folder, but has no way to detect changed files (same filename, updated content). It also has no protection against Cloudflare 403 storms — a single block can cause hundreds of individual timeouts before giving up.

Solution

New src/syncmanager.py module with a SyncManager class that:

1. Delta sync via GitHub Tree API — fetches the SHA1 + file size for every file in STOCD/SETS-Data, compares against local files, and downloads only what changed or is missing. The tree response is cached for 1 hour so repeated startups don't hit the API every time.

2. Circuit breaker for 403 errors — after 3× HTTP 403 from GitHub or stowiki, the source is disabled for the rest of the session. Prevents Cloudflare stall storms where hundreds of requests queue up and time out one by one.

3. Retry + stall timeout — transient network errors are retried once with a 10 s stall timeout per request.

4. Ship image wiki fallback — if GitHub returns 404 for a ship image, falls back to stowiki Special:FilePath (same behaviour as Downloader, now with circuit breaker + retry).

Integration

• SyncManager is instantiated in SETS.__init__ alongside Downloader (2 lines)
• populate_cache() calls sync.run() before load_cargo_cache(), so cargo JSON files on disk are fresh before the 7-day TTL check runs — avoiding unnecessary stowiki cargo fetches on a healthy install
• ImageManager.download_images() is unchanged: it continues to handle wiki-only groups (boff abilities, skill icons) not covered by the GitHub tree

No new dependencies

syncmanager.py only uses the standard library (hashlib, queue, threading) and requests (already a dependency via Downloader).