fix: track backend/uv.lock for reproducible CI builds

backend/uv.lock was gitignored, causing CI to resolve dependencies
fresh and install ruff 0.15.0 instead of the pinned 0.14.14. This
led to formatting check failures due to formatting differences
between ruff versions. All other project lockfiles (aggregator,
sdk/python, syfthub-api, mcp) were already tracked.

Author: IonesioJunior

.gitignore

@@ -154,7 +154,6 @@ Thumbs.db
 ehthumbs.db
 
 # uv
-backend/uv.lock
 
 # ruff
 .ruff_cache/