feat: subpath-aware routing in Go server

[wesm]

The Go server currently serves at /. Subpath stripping is handled by the deployer's reverse proxy (e.g. Caddy handle_path).

Native subpath support in the Go server would simplify deployment for users who want to serve agentsview under a prefix path without requiring proxy-level path rewriting.

Tracked from the pg-sync refactor spec (out of scope for initial implementation).