docs: add doc.go files and godoc comments to all packages

Add package-level documentation (doc.go) to all 22 first-party
packages and godoc comments to ~150 previously undocumented
exported symbols across common/, network/, and server/.

server/api/api_server.go

@@ -15,6 +15,7 @@ import (
 	"go.uber.org/zap"
 )
 
+// Config holds the dependencies required to initialize an APIServer.
 type Config struct {
 	Logger      *zap.Logger
 	DB          *sqlx.DB