Add Image Hash filter list

[jb3]

This PR migrates image scam detection from the hardcoded unique image filter to a dynamic image_hash filter list.

• Added a new image_hash filter type and image_hash deny list flow in filtering.
• Removed the unique image filter implementation
• Added shared Rhodium hash helpers and updated hash parsing to use Rhodium's hex response key.
• Added a moderator utility command: !filter imagehash (alias: !filter hash) to hash an attached image and return both hex and i64 hash
• Updated image hash alert formatting so #mod-alert output always includes a description (with fallback text if missing)

A manual migration of the currently known image hashes is required after deploy. Hashes must be added as image_hash deny filters (this time using hex values instead of the integer hashes).