chore: complete rename from go-upkeep to uptop

- Module path: gitea.lerkolabs.com/lerko/uptop
- Binary: cmd/uptop/
- All imports updated to full module path
- Env vars: UPKEEP_* → UPTOP_*
- Prometheus metrics: upkeep_* → uptop_*
- Default DB: uptop.db
- Docker image: lerko/uptop
- All docs, compose files, CI updated

Only remaining "go-upkeep" reference is the fork attribution in README.

CONTRIBUTING.md

@@ -3,7 +3,7 @@
 ## Development
 
 ```sh
-go run cmd/goupkeep/main.go -demo  # starts with sample data
+go run cmd/uptop/main.go -demo  # starts with sample data
 ssh -p 23234 localhost              # connect to TUI
 ```