WIP: Add sandbox proxy catch-all to control plane

Add SandboxProxyWrapper that intercepts requests with Host headers
matching {port}-{sandbox_id}.{domain} and proxies them through the
owning host agent's /proxy endpoint.

Authentication is via X-API-Key only (no JWT). The API key's team must
own the sandbox. Export EnsureScheme from lifecycle package for reuse.

Request flow: SDK -> Caddy -> CP catch-all -> Host Agent -> sandbox VM.

This is an intermediate state — needs further work for the full code
interpreter feature.

internal/api/handler_sandbox_proxy.go

@@ -0,0 +1,149 @@
+package api
+
+import (
+	"fmt"
+	"log/slog"
+	"net/http"
+	"net/http/httputil"
+	"net/url"
+	"regexp"
+	"strconv"
+	"strings"
+
+	"git.omukk.dev/wrenn/sandbox/internal/auth"
+	"git.omukk.dev/wrenn/sandbox/internal/db"
+	"git.omukk.dev/wrenn/sandbox/internal/lifecycle"
+)
+
+// sandboxHostPattern matches hostnames like "49999-sb-abcd1234.localhost" or
+// "49999-sb-abcd1234.example.com". Captures: port, sandbox ID.
+var sandboxHostPattern = regexp.MustCompile(`^(\d+)-(sb-[0-9a-f]+)\.`)
+
+// SandboxProxyWrapper wraps an existing HTTP handler and intercepts requests
+// whose Host header matches the {port}-{sandbox_id}.{domain} pattern. Matching
+// requests are reverse-proxied through the host agent that owns the sandbox.
+// All other requests are passed through to the inner handler.
+//
+// Authentication is via X-API-Key header only (no JWT). The API key's team
+// must own the sandbox.
+type SandboxProxyWrapper struct {
+	inner     http.Handler
+	db        *db.Queries
+	pool      *lifecycle.HostClientPool
+	transport http.RoundTripper
+}
+
+// NewSandboxProxyWrapper creates a new proxy wrapper.
+func NewSandboxProxyWrapper(inner http.Handler, queries *db.Queries, pool *lifecycle.HostClientPool) *SandboxProxyWrapper {
+	return &SandboxProxyWrapper{
+		inner:     inner,
+		db:        queries,
+		pool:      pool,
+		transport: http.DefaultTransport,
+	}
+}
+
+func (h *SandboxProxyWrapper) ServeHTTP(w http.ResponseWriter, r *http.Request) {
+	host := r.Host
+	// Strip port from Host header (e.g. "49999-sb-abcd1234.localhost:8000" → "49999-sb-abcd1234.localhost")
+	if colonIdx := strings.LastIndex(host, ":"); colonIdx != -1 {
+		host = host[:colonIdx]
+	}
+
+	matches := sandboxHostPattern.FindStringSubmatch(host)
+	if matches == nil {
+		h.inner.ServeHTTP(w, r)
+		return
+	}
+
+	port := matches[1]
+	sandboxID := matches[2]
+
+	// Validate port.
+	portNum, err := strconv.Atoi(port)
+	if err != nil || portNum < 1 || portNum > 65535 {
+		http.Error(w, "invalid port", http.StatusBadRequest)
+		return
+	}
+
+	// Authenticate: require API key or JWT, extract team ID.
+	teamID, err := h.authenticateRequest(r)
+	if err != nil {
+		writeError(w, http.StatusUnauthorized, "unauthorized", err.Error())
+		return
+	}
+
+	ctx := r.Context()
+
+	// Look up sandbox and verify ownership.
+	sb, err := h.db.GetSandboxByTeam(ctx, db.GetSandboxByTeamParams{
+		ID:     sandboxID,
+		TeamID: teamID,
+	})
+	if err != nil {
+		http.Error(w, "sandbox not found", http.StatusNotFound)
+		return
+	}
+
+	if sb.Status != "running" {
+		http.Error(w, fmt.Sprintf("sandbox is not running (status: %s)", sb.Status), http.StatusConflict)
+		return
+	}
+
+	agentHost, err := h.db.GetHost(ctx, sb.HostID)
+	if err != nil {
+		http.Error(w, "host agent not found", http.StatusServiceUnavailable)
+		return
+	}
+
+	if !agentHost.Address.Valid || agentHost.Address.String == "" {
+		http.Error(w, "host agent has no address", http.StatusServiceUnavailable)
+		return
+	}
+
+	agentAddr := lifecycle.EnsureScheme(agentHost.Address.String)
+	upstreamPath := fmt.Sprintf("/proxy/%s/%s%s", sandboxID, port, r.URL.Path)
+
+	target, err := url.Parse(agentAddr)
+	if err != nil {
+		http.Error(w, "invalid host agent address", http.StatusInternalServerError)
+		return
+	}
+
+	proxy := &httputil.ReverseProxy{
+		Transport: h.transport,
+		Director: func(req *http.Request) {
+			req.URL.Scheme = target.Scheme
+			req.URL.Host = target.Host
+			req.URL.Path = upstreamPath
+			req.URL.RawQuery = r.URL.RawQuery
+			req.Host = target.Host
+		},
+		ErrorHandler: func(w http.ResponseWriter, r *http.Request, err error) {
+			slog.Debug("sandbox proxy error",
+				"sandbox_id", sandboxID,
+				"port", port,
+				"error", err,
+			)
+			http.Error(w, "proxy error: "+err.Error(), http.StatusBadGateway)
+		},
+	}
+
+	proxy.ServeHTTP(w, r)
+}
+
+// authenticateRequest validates the request's API key and returns the team ID.
+// Only API key authentication is supported for sandbox proxy requests (not JWT).
+func (h *SandboxProxyWrapper) authenticateRequest(r *http.Request) (string, error) {
+	key := r.Header.Get("X-API-Key")
+	if key == "" {
+		return "", fmt.Errorf("X-API-Key header required")
+	}
+
+	hash := auth.HashAPIKey(key)
+	row, err := h.db.GetAPIKeyByHash(r.Context(), hash)
+	if err != nil {
+		return "", fmt.Errorf("invalid API key")
+	}
+	return row.TeamID, nil
+}