Improve Docker image tagging strategy and remove Swagger version

- Replace 'stable' tag with semver-based tags (1.2.3, 1.2, 1)
- Use 'edge' tag for main branch instead of 'latest'
- Reserve 'latest' tag only for tagged releases
- Remove version from Swagger docs (tracked via git tags instead)
- Document available image tags in README

cmd/api/main.go

@@ -18,7 +18,6 @@ import (
 )
 
 // @title Apocapoc API
-// @version 1.0
 // @description Self-hosted habit tracking service
 // @termsOfService http://swagger.io/terms/