hotfix(paliadin): ship user_id on /chat/turn (aichat tenant-DB requirement)

m reported "ai chat seems not to be wired anymore" + the frontend
showed "Verbindung verloren. Antwort wird nachgereicht…".

Root cause: aichat on mRiver added a tenant-DB layer that demands
`user_id` on every /chat/turn request:

  {"error":{"code":"bad_request",
            "message":"user_id is required when a tenant DB is
                       configured","retryable":false}}

aichat itself is healthy (/chat/health 200, paliadin session ok:true,
last successful turn was ~2.6h ago). The paliad side built and shipped
an aichatTurnRequest without user_id, so every turn since the tenant-DB
flip 400s; paliad's SSE relay receives no upstream data and closes
empty, producing the user-visible "Verbindung verloren".

Fix: add UserID to aichatTurnRequest (json: user_id, mandatory now),
populate from req.UserID.String() at the call site. The userID was
already in scope (used for JWT mint + username lookup); the struct just
wasn't shipping it.

Regression test in TestRunTurn_HappyPath_ViaCallHTTP asserts
captured.UserID == request UUID so a future struct edit that drops the
field fails CI instead of production.

internal/services/aichat_paliadin.go

@@ -217,6 +217,7 @@ func (s *AichatPaliadinService) RunTurn(ctx context.Context, req TurnRequest) (*
 	body := aichatTurnRequest{
 		Persona:   s.cfg.Persona,
 		Username:  username,
+		UserID:    req.UserID.String(),
 		SessionID: req.SessionID,
 		Message:   sanitiseForTmux(req.UserMessage),
 		JWT:       jwt,
@@ -611,8 +612,13 @@ func (s *AichatPaliadinService) clearPrimed(session string) {
 // =============================================================================
 
 type aichatTurnRequest struct {
-	Persona   string                 `json:"persona"`
+	Persona  string `json:"persona"`
-	Username  string                 `json:"username"`
+	Username string `json:"username"`
+	// UserID is the paliad user UUID, required by aichat now that a
+	// tenant DB is configured ("user_id is required when a tenant DB
+	// is configured"). Without it /chat/turn 400s and the SSE relay
+	// closes empty → "Verbindung verloren" on the frontend.
+	UserID    string                 `json:"user_id"`
 	SessionID string                 `json:"session_id,omitempty"`
 	Message   string                 `json:"message"`
 	JWT       string                 `json:"jwt,omitempty"`

internal/services/aichat_paliadin_test.go

@@ -501,6 +501,7 @@ func TestRunTurn_HappyPath_ViaCallHTTP(t *testing.T) {
 	body := aichatTurnRequest{
 		Persona:  s.cfg.Persona,
 		Username: s.usernameFor(context.Background(), uid),
+		UserID:   uid.String(),
 		Message:  "Hello",
 		JWT:      jwtTok,
 		Meta:     buildAichatMeta(TurnRequest{PageOrigin: "/dashboard"}),
@@ -516,6 +517,12 @@ func TestRunTurn_HappyPath_ViaCallHTTP(t *testing.T) {
 	if captured.Username != "user-aaaaaaaa" {
 		t.Errorf("username = %q; want user-aaaaaaaa (nil DB fallback)", captured.Username)
 	}
+	// Regression for the 2026-05-21 outage: aichat now requires user_id
+	// when a tenant DB is configured; missing → 400 → SSE drop on the
+	// frontend ("Verbindung verloren"). The struct must carry it.
+	if captured.UserID != uid.String() {
+		t.Errorf("user_id = %q; want %q", captured.UserID, uid.String())
+	}
 	if captured.Message != "Hello" {
 		t.Errorf("message = %q; want Hello", captured.Message)
 	}