feat(sidecar): scoped PhotoPrism-compatible API proxy for third-party apps

PhotoPrism CE doesn't enforce auth_users.base_path on API reads (any
user can q=path:"other/*"). New /api/v1/* proxy forwards to PhotoPrism
with per-session enforcement: search queries get their path filter
validated/injected, single-photo reads and like are ownership-checked,
hash-addressed media and session/config pass through, everything else
is 403 for scoped users. Admins (empty BasePath) pass through fully.
prism.hubris.network will route here instead of straight to PhotoPrism.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

sidecar/handlers_ppproxy_test.go

@@ -0,0 +1,72 @@
+package main
+
+import (
+	"net/url"
+	"testing"
+)
+
+func TestScopeQ(t *testing.T) {
+	cases := []struct {
+		name string
+		q    string
+		base string
+		want string
+	}{
+		{"empty q gains scope", "", "dtoro", `path:"dtoro/*"`},
+		{"plain search gains scope", "label:dog", "dtoro", `label:dog path:"dtoro/*"`},
+		{"inside path kept", `path:"dtoro/2024/*" label:dog`, "dtoro", `path:"dtoro/2024/*" label:dog`},
+		{"exact base kept", `path:"dtoro"`, "dtoro", `path:"dtoro"`},
+		{"outside path replaced", `path:"muli/*"`, "dtoro", `path:"dtoro/*"`},
+		{"bare term outside replaced", `path:muli/x label:dog`, "dtoro", `label:dog path:"dtoro/*"`},
+		{"pipe alternative escaping", `path:"dtoro/*|muli/*"`, "dtoro", `path:"dtoro/*"`},
+		{"pipe all inside kept", `path:"dtoro/a|dtoro/b/*"`, "dtoro", `path:"dtoro/a|dtoro/b/*"`},
+		{"sibling prefix rejected", `path:"dtoro2/*"`, "dtoro", `path:"dtoro/*"`},
+		{"bare wildcard on base rejected", `path:dtoro*`, "dtoro", `path:"dtoro/*"`},
+		{"mixed valid+invalid terms collapse", `path:"dtoro/a" path:"muli/b"`, "dtoro", `path:"dtoro/*"`},
+		{"case-insensitive filter name", `PATH:"muli/*"`, "dtoro", `path:"dtoro/*"`},
+		{"nested base", `path:"family/alice/x"`, "family/alice", `path:"family/alice/x"`},
+		{"nested base parent escape", `path:"family/*"`, "family/alice", `path:"family/alice/*"`},
+	}
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			if got := scopeQ(tc.q, tc.base); got != tc.want {
+				t.Errorf("scopeQ(%q, %q) = %q, want %q", tc.q, tc.base, got, tc.want)
+			}
+		})
+	}
+}
+
+func TestScopeSearchValues(t *testing.T) {
+	v := url.Values{}
+	v.Set("count", "60")
+	v.Set("path", "muli/*")
+	got := scopeSearchValues(v, "dtoro")
+	if got.Get("path") != "" {
+		t.Errorf("outside path param should be dropped, got %q", got.Get("path"))
+	}
+	if got.Get("q") != `path:"dtoro/*"` {
+		t.Errorf("q should carry the scope, got %q", got.Get("q"))
+	}
+	if got.Get("count") != "60" {
+		t.Errorf("unrelated params must survive, got count=%q", got.Get("count"))
+	}
+
+	v2 := url.Values{}
+	v2.Set("path", "dtoro/2024")
+	got2 := scopeSearchValues(v2, "dtoro")
+	if got2.Get("path") != "dtoro/2024" {
+		t.Errorf("inside path param should be kept, got %q", got2.Get("path"))
+	}
+}
+
+func TestPathValueAllowed(t *testing.T) {
+	if pathValueAllowed(`"muli/*"`, "dtoro") {
+		t.Error("outside value must be rejected")
+	}
+	if !pathValueAllowed(`"dtoro/Photos/2024"`, "dtoro") {
+		t.Error("inside value must be allowed")
+	}
+	if pathValueAllowed("dtoro/a|muli/b", "dtoro") {
+		t.Error("any escaping pipe alternative must reject the whole value")
+	}
+}