fix(paliadin): fall back to one-shot when aichat persona lacks streaming

Symptom: paliadin chat returns "Verbindung verloren" because aichat's
paliadin persona is not configured with streaming support — every
RunTurnStream() call gets back HTTP 400 unsupported_streaming and the
SSE stream closes empty.

Fix: when RunTurnStream() detects "unsupported_streaming" in the
upstream error, transparently retry against /chat/turn (non-streaming)
with the same body. The full response gets emitted as a single
StreamChunk + StreamMeta so the SSE relay sees identical event
ordering. Persistence (completeTurn + markPrimed) mirrors the one-shot
RunTurn() path.

No real-time chunking until the persona is reconfigured upstream, but
the chat works end-to-end. Once the paliadin persona supports streaming
on aichat, this code path goes dormant — the unsupported_streaming
branch is only entered when the upstream actually returns that error.

Diagnostic logs from commit 937ff13 made this visible:
  paliadin: backend returned error err=aichat: HTTP 400 (bad_request):
  unsupported_streaming: persona paliadin does not support streaming

Refs m/paliad demo path.

internal/services/aichat_paliadin_stream.go

@@ -220,6 +220,14 @@ func (s *AichatPaliadinService) RunTurnStream(ctx context.Context, req TurnReque
 	}
 
 	if streamErr != nil {
+		// Aichat persona without streaming support — graceful fallback to
+		// the one-shot /chat/turn endpoint. Same body shape; we adapt the
+		// non-streaming response into a single StreamChunk so the caller
+		// sees identical event ordering.
+		if strings.Contains(streamErr.Error(), "unsupported_streaming") {
+			log.Printf("paliadin: persona %q lacks streaming support — falling back to one-shot turn %s", s.cfg.Persona, turnID)
+			return s.fallbackOneShotFromStream(ctx, turnID, body, events, startedAt, session)
+		}
 		// Don't overwrite an existing error_code we may have set above.
 		_ = s.markTurnError(ctx, turnID, classifyAichatError(streamErr))
 		return nil, streamErr
@@ -255,6 +263,80 @@ func (s *AichatPaliadinService) RunTurnStream(ctx context.Context, req TurnReque
 	}, nil
 }
 
+// fallbackOneShotFromStream runs the same `body` against aichat's
+// non-streaming /chat/turn endpoint and adapts the response into the
+// StreamingPaliadin contract — a single StreamChunk + StreamMeta +
+// StreamConversation, followed by `events` being closed by the
+// outer RunTurnStream's defer. Used when the configured persona doesn't
+// support streaming (aichat returns HTTP 400 unsupported_streaming).
+//
+// Identical persistence shape as the one-shot RunTurn: completeTurn +
+// markPrimed/clearPrimed. No new turn row (already inserted by
+// RunTurnStream). No primer rebuild (already in body).
+func (s *AichatPaliadinService) fallbackOneShotFromStream(
+	ctx context.Context,
+	turnID uuid.UUID,
+	body aichatTurnRequest,
+	events chan<- StreamEvent,
+	startedAt time.Time,
+	session string,
+) (*TurnResult, error) {
+	var resp aichatTurnResponse
+	if err := s.callHTTP(ctx, http.MethodPost, "/chat/turn", body, &resp); err != nil {
+		_ = s.markTurnError(ctx, turnID, classifyAichatError(err))
+		safeSendStream(ctx, events, StreamEvent{
+			Kind:    StreamError,
+			Code:    classifyAichatError(err),
+			Message: err.Error(),
+		})
+		return nil, err
+	}
+
+	if resp.PaneSpawned {
+		s.clearPrimed(session)
+	} else {
+		s.markPrimed(session)
+	}
+
+	cleanBody := resp.Response
+	tokens := approxTokenCount(cleanBody)
+	chipCount := countChips(cleanBody)
+	finished := time.Now().UTC()
+	durationMS := int(finished.Sub(startedAt) / time.Millisecond)
+
+	tmeta := trailerMeta{
+		UsedTools:     resp.Meta.UsedTools,
+		ClassifierTag: resp.Meta.ClassifierTag,
+		RowsSeen:      coerceAichatRowsSeen(resp.Meta.RowsSeen),
+	}
+
+	// Emit the response as a single chunk so the frontend renders it.
+	safeSendStream(ctx, events, StreamEvent{
+		Kind:    StreamChunk,
+		Content: cleanBody,
+	})
+	safeSendStream(ctx, events, StreamEvent{
+		Kind:          StreamMeta,
+		UsedTools:     tmeta.UsedTools,
+		ClassifierTag: tmeta.ClassifierTag,
+		RowsSeen:      tmeta.RowsSeen,
+	})
+
+	if err := s.completeTurn(ctx, turnID, finished, durationMS, cleanBody, tokens, tmeta, chipCount); err != nil {
+		log.Printf("paliadin: complete turn %s (fallback one-shot): %v", turnID, err)
+	}
+
+	return &TurnResult{
+		TurnID:        turnID,
+		Response:      cleanBody,
+		UsedTools:     tmeta.UsedTools,
+		RowsSeen:      tmeta.RowsSeen,
+		ChipCount:     chipCount,
+		ClassifierTag: tmeta.ClassifierTag,
+		DurationMS:    durationMS,
+	}, nil
+}
+
 // streamFrame is one decoded SSE event.
 type streamFrame struct {
 	event     string // "" → default (data:) event