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.

Makefile

@@ -41,7 +41,7 @@ GO_LICENSES_PACKAGE ?= github.com/google/go-licenses@v1
 GOVULNCHECK_PACKAGE ?= golang.org/x/vuln/cmd/govulncheck@v1
 ACTIONLINT_PACKAGE ?= github.com/rhysd/actionlint/cmd/actionlint@v1.7.9
 
-DOCKER_IMAGE ?= gitea/gitea
+DOCKER_IMAGE ?= gitcaddy/gitcaddy-server
 DOCKER_TAG ?= latest
 DOCKER_REF := $(DOCKER_IMAGE):$(DOCKER_TAG)
 
@@ -70,10 +70,10 @@ else ifeq ($(patsubst Windows%,Windows,$(OS)),Windows)
 endif
 ifeq ($(IS_WINDOWS),yes)
 	GOFLAGS := -v -buildmode=exe
-	EXECUTABLE ?= gitea.exe
+	EXECUTABLE ?= gitcaddy-server.exe
 else
 	GOFLAGS := -v
-	EXECUTABLE ?= gitea
+	EXECUTABLE ?= gitcaddy-server
 endif
 
 ifeq ($(shell sed --version 2>/dev/null | grep -q GNU && echo gnu),gnu)