Hash validation, archive cleanup and download cache refactor

[Tosox]

• Add optional hash field to mod entries for post-download integrity checks
• Scrape MD5 from ModDB addon page to skip re-downloads when server file is unchanged
• Collect hash mismatches throughout install; present summary with option to write actual hashes back into modlist.yaml
• Add archive cleanup strategy (KEEP_ALL, KEEP_LATEST_ONLY, DELETE_ALL) to user config; default KEEP_LATEST_ONLY
• Decouple download cache check from FilenameResolver; resolver only called when an actual download is needed
• Introduce downloads manifest (manifest.json) to track per-mod download and install state; skip reinstall when installed hash matches archive hash

Closes #65, #67, #72