feat(gateway): /g/:slug proxy data plane with key auth and rate limits (#66)

The admin surface under /admin/gateway managed api_services and api_keys,
but no public route ever consumed them. Mounts /g/:slug and /g/:slug/* on
the root router (outside /api/v1 so route_prefix stays clean) and runs the
full pipeline: slug lookup -> X-API-Key/Bearer extraction -> prefix-indexed
bcrypt key auth -> allowed_service_ids scope -> per-minute buckets (key and
service caps) -> monthly usage_counters quotas -> gateway.ProxyRequest with
per-service timeout and upstream auth headers -> usage/metrics/incident
recording.

Live-verified: proxied GET/POST, 401/404/429 boundaries, counter upserts,
UPSTREAM_5XX incidents. Unit tests cover URL building, the rpm bucket, key
extraction, and the service allowlist.

Generated with [Devin](https://devin.ai)

Co-authored-by: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

app/backend/internal/gateway/proxy_test.go

@@ -0,0 +1,64 @@
+package gateway
+
+import "testing"
+
+func TestBuildTargetURL(t *testing.T) {
+	cases := []struct {
+		name                                       string
+		upstream, prefix, path, query, want        string
+	}{
+		{
+			name:     "strips route prefix",
+			upstream: "https://api.internal:8080",
+			prefix:   "/g/weather",
+			path:     "/g/weather/v1/current",
+			query:    "city=oslo",
+			want:     "https://api.internal:8080/v1/current?city=oslo",
+		},
+		{
+			name:     "prefix root maps to upstream root",
+			upstream: "https://api.internal",
+			prefix:   "/g/weather",
+			path:     "/g/weather",
+			want:     "https://api.internal/",
+		},
+		{
+			name:     "upstream base path is preserved",
+			upstream: "https://api.internal/base",
+			prefix:   "/g/x",
+			path:     "/g/x/ping",
+			want:     "https://api.internal/base/ping",
+		},
+		{
+			name:     "root prefix leaves path untouched",
+			upstream: "https://api.internal",
+			prefix:   "/",
+			path:     "/g/raw",
+			want:     "https://api.internal/g/raw",
+		},
+		{
+			name:     "path without prefix is still proxied",
+			upstream: "https://api.internal",
+			prefix:   "/g/x",
+			path:     "/other",
+			want:     "https://api.internal/other",
+		},
+	}
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			got, err := BuildTargetURL(tc.upstream, tc.prefix, tc.path, tc.query)
+			if err != nil {
+				t.Fatalf("unexpected error: %v", err)
+			}
+			if got != tc.want {
+				t.Fatalf("got %q, want %q", got, tc.want)
+			}
+		})
+	}
+}
+
+func TestBuildTargetURL_InvalidUpstream(t *testing.T) {
+	if _, err := BuildTargetURL("://bad", "/g/x", "/g/x/y", ""); err == nil {
+		t.Fatal("expected error for unparsable upstream")
+	}
+}