feat: rebrand to GitCaddy Server

Major rebrand from Gitea to GitCaddy Server:

Binary & Package:
- Rename binary from gitea to gitcaddy-server
- Update Makefile EXECUTABLE variable
- Update Docker image name to gitcaddy/gitcaddy-server
- Update release artifact names

Environment Variables (backward compatible):
- Add GITCADDY__ prefix for config env vars
- Add GITCADDY_WORK_DIR and GITCADDY_CUSTOM
- Keep GITEA__ and GITEA_* as fallback for compatibility

UI Branding:
- Update footer to Powered by GitCaddy Server
- Add Based on Gitea attribution in footer
- Update Swagger API title and description
- Update webhook placeholders

Code References:
- Update cmd/main.go app name, usage, description
- Update error messages in setting.go
- Add based_on locale key

This rebrand maintains full backward compatibility with existing
Gitea installations while establishing GitCaddy Server identity.

README.md

@@ -264,7 +264,7 @@ Download from [Releases](https://git.marketally.com/gitcaddy/gitea/releases):
 
 ```bash
 # Linux (amd64)
-curl -L -o gitcaddy https://git.marketally.com/gitcaddy/gitea/releases/latest/download/gitea-linux-amd64
+curl -L -o gitcaddy-server https://git.marketally.com/gitcaddy/gitea/releases/latest/download/gitcaddy-server-linux-amd64
 chmod +x gitcaddy
 ./gitcaddy web
 ```
@@ -275,7 +275,7 @@ chmod +x gitcaddy
 git clone https://git.marketally.com/gitcaddy/gitea.git
 cd gitea
 TAGS="bindata sqlite sqlite_unlock_notify" make build
-./gitea web
+./gitcaddy-server web
 ```
 
 ### Docker