feat(server): HMAC stream token auth path (UPnP slice 1/6)

Adds SignStreamToken / VerifyStreamToken (HMAC-SHA256 over
trackID|exp) and modifies handleGetStream to accept either the
existing session cookie OR a valid signed token. Stream route
moved out of the authed group so the handler's own auth check
runs and the token bypass is reachable.

Enables Sonos / UPnP speakers to fetch the stream URL without
carrying the user's session cookie - they cannot. The token is
short-lived (max 24h per the design); expiry checked at request
time only, not per-byte, so long tracks play through.

streamSecret field on handlers is nil for now; Task 2 wires the
loader (env var with auto-generated fallback persisted in
app_preferences).

Adds auth.OptionalUser - the permissive sibling of RequireUser
that attaches the user to context when a valid cookie / bearer is
present but does NOT 401 on absence. The stream route is wrapped
with it so the handler can fall through to the token path when
no session is present.

newLibraryRouter (test fixture) gets a synthetic-user middleware
on the stream route so existing media_test tests keep passing
without seeding a real session row - production traffic uses
auth.OptionalUser, the test path uses auth.UserCtxKeyForTest().

Five tests cover round-trip, tampered token rejection, expiry,
wrong-track-ID, and wrong-secret rejection. CI verifies.

internal/api/api.go

@@ -54,6 +54,17 @@ func Mount(r chi.Router, pool *pgxpool.Pool, logger *slog.Logger, events *playev
 		api.Post("/auth/register", h.handleRegister)
 		api.Post("/auth/forgot-password", h.handleForgotPassword)
 		api.Post("/auth/reset-password", h.handleResetPassword)
+
+		// Stream lives outside authed.Group so it can accept EITHER a
+		// session (resolved by the OptionalUser middleware) OR a signed
+		// query token (UPnP / Sonos path; see streamAuthOk). The
+		// middleware attaches user to context when a valid cookie /
+		// bearer is present but does NOT 401 on absence; the handler's
+		// own streamAuthOk performs the actual auth check. See the
+		// design at
+		// docs/superpowers/specs/2026-06-03-android-output-picker-upnp-design.md.
+		api.With(auth.OptionalUser(pool, logger)).Get("/tracks/{id}/stream", h.handleGetStream)
+
 		api.Group(func(authed chi.Router) {
 			authed.Use(auth.RequireUser(pool))
 			authed.Post("/auth/logout", h.handleLogout)
@@ -77,7 +88,8 @@ func Mount(r chi.Router, pool *pgxpool.Pool, logger *slog.Logger, events *playev
 			authed.Get("/library/albums", h.handleListLibraryAlbums)
 			authed.Get("/library/sync", h.handleLibrarySync)
 			authed.Get("/tracks/{id}", h.handleGetTrack)
-			authed.Get("/tracks/{id}/stream", h.handleGetStream)
+			// /tracks/{id}/stream is mounted above with OptionalUser so
+			// it can accept either a session or a signed token.
 			authed.Get("/search", h.handleSearch)
 			authed.Get("/radio", h.handleRadio)
 			authed.Get("/discover/suggestions", h.handleListSuggestions)
@@ -205,4 +217,13 @@ type handlers struct {
 	mailer            mailer.Sender
 	eventbus          *eventbus.Bus
 	playlistScheduler *playlists.Scheduler
+	// streamSecret is the HMAC key used by SignStreamToken /
+	// VerifyStreamToken to authenticate the UPnP-speaker stream path
+	// (see internal/api/stream_token.go and the design at
+	// docs/superpowers/specs/2026-06-03-android-output-picker-upnp-design.md).
+	// nil in slice 1; slice 2 wires the env-var-with-app_preferences-
+	// fallback loader. A nil secret leaves the cookie path intact and
+	// makes the token path unreachable (HMAC of empty key won't match
+	// anything a client mints), which is the desired slice-1 default.
+	streamSecret []byte
 }