refactor: eliminate internal/domain package, colocate types with implementations

Move ImageID and ContainerID into internal/docker/types.go (where they're
primarily used) and UnparsedURL into internal/service/webhook/types.go.

This follows the Go convention of defining types alongside the code that
uses them rather than in a separate 'domain' package.

internal/service/webhook/types.go

@@ -0,0 +1,7 @@
+package webhook
+
+// UnparsedURL is a URL stored as a plain string without parsing.
+// Use this instead of string when the value is known to be a URL
+// but should not be parsed into a net/url.URL (e.g. webhook URLs,
+// compare URLs from external payloads).
+type UnparsedURL string