refactor(repo): consolidate openflare-server to root and move subprojects to internal/apps

- Merge all files inside openflare-server to the repository root directory.
- Relocate agent, relay, and flared subprojects from internal/ to internal/apps/.
- Combine docker-compose files and update build context paths to root.
- Update GitHub workflows and Dockerfiles to refer to new directories and package names.
- Rewrite Go package imports across all files.
- Resolve database renew test race condition and clean up docs.

internal/apps/agent/updater/restart_unix.go

@@ -0,0 +1,51 @@
+//go:build !windows
+
+package updater
+
+import (
+	"fmt"
+	"log/slog"
+	"os"
+	"syscall"
+)
+
+func replaceAndRestart(execPath string, tmpPath string) error {
+	backupPath := execPath + ".bak"
+	if err := removeBackupBinary(backupPath); err != nil {
+		return err
+	}
+	if err := os.Rename(execPath, backupPath); err != nil {
+		renameErr := err
+		if err := os.Remove(tmpPath); err != nil && !os.IsNotExist(err) {
+			slog.Error("remove tmp binary failed", "path", tmpPath, "error", err)
+			return fmt.Errorf("backup current binary: %w; remove tmp binary: %v", renameErr, err)
+		}
+		return fmt.Errorf("backup current binary: %w", renameErr)
+	}
+	if err := os.Rename(tmpPath, execPath); err != nil {
+		replaceErr := err
+		if err := os.Rename(backupPath, execPath); err != nil {
+			slog.Error("restore backup binary failed", "path", backupPath, "error", err)
+			return fmt.Errorf("replace binary: %w; restore backup binary: %v", replaceErr, err)
+		}
+		return fmt.Errorf("replace binary: %w", replaceErr)
+	}
+	if err := removeBackupBinary(backupPath); err != nil {
+		return err
+	}
+	if err := syscall.Exec(execPath, os.Args, os.Environ()); err != nil {
+		return fmt.Errorf("exec restart: %w", err)
+	}
+	return fmt.Errorf("unreachable after exec")
+}
+
+func removeBackupBinary(path string) error {
+	if err := os.Remove(path); err != nil {
+		if os.IsNotExist(err) {
+			return nil
+		}
+		slog.Error("remove backup binary failed", "path", path, "error", err)
+		return err
+	}
+	return nil
+}