[100 MRG] feat: add emboss filter (Closes #4)

[laurentketterle-hub]

Summary

Add emboss filter to complete the filters pack requested in #4.

Changes

• ops.py: Add function using Pillow
• backend/base.py: Add abstract method
• backend/mock.py: Add mock implementation
• backend/live.py: Add live implementation via Pillow
• server.py: Add MCP tool

Testing

• Uses Pillow's built-in emboss filter — consistent across mock/live
• Works with existing / infrastructure

Fixes #4

Signed-off-by: laurentketterle-hub laurentketterle-hub@users.noreply.github.com

[laurentketterle-hub]

🤖 @code-cannot-be-blank CI ALL CLEAN ✅ — PR is mergeable:clean. Ready for MERGE + SETTLE on-chain. Please review and merge. 🙏