fix(api): Langfuse v4.x API 相容性修復

Langfuse SDK v4.0.1 API 變更:
- 移除 client.trace() 方法
- 改用 create_trace_id() + OTEL 整合

修復:
- __enter__: 檢查 trace() 方法存在再使用，否則用 create_trace_id()
- generation/span/score: 加入 hasattr 檢查，v4 改用 debug log

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

apps/api/src/services/langfuse_client.py

@@ -123,14 +123,27 @@ class LangfuseTraceContext:
                         self._otel_trace_id
                     )
 
-                self.trace = self._client.trace(
-                    name=self.name,
-                    metadata=enriched_metadata,
-                )
-
-                # 記錄 Langfuse trace_id 供反向連結
-                if self.trace:
-                    self._langfuse_trace_id = self.trace.id
+                # Langfuse v4.x API: 使用 create_trace_id() 取代舊的 trace()
+                # 舊 API (v2.x): self._client.trace(name=..., metadata=...)
+                # 新 API (v4.x): 透過 OTEL 整合或裝飾器
+                if hasattr(self._client, "trace"):
+                    # Legacy API (v2.x)
+                    self.trace = self._client.trace(
+                        name=self.name,
+                        metadata=enriched_metadata,
+                    )
+                    if self.trace:
+                        self._langfuse_trace_id = self.trace.id
+                else:
+                    # Langfuse v4.x - 使用 create_trace_id() 並記錄
+                    # 完整追蹤依賴 OTEL 整合
+                    self._langfuse_trace_id = self._client.create_trace_id()
+                    logger.debug(
+                        "langfuse_v4_trace_created",
+                        trace_id=self._langfuse_trace_id,
+                        name=self.name,
+                        note="Full tracing via OTEL integration",
+                    )
 
                 logger.debug(
                     "langfuse_trace_started",
@@ -177,25 +190,33 @@ class LangfuseTraceContext:
             usage: Token 使用量 {"input": x, "output": y}
             metadata: 額外 metadata
         """
-        if not self.trace:
-            return None
-
-        try:
-            gen = self.trace.generation(
-                name=name,
-                model=model,
-                input=input,
-                output=output,
-                usage=usage,
-                metadata=metadata or {},
-            )
-            return gen
-        except Exception as e:
-            logger.warning(
-                "langfuse_generation_failed",
-                error=str(e),
+        # Langfuse v4.x: trace 物件可能不存在，改用 logger 記錄
+        if self.trace and hasattr(self.trace, "generation"):
+            try:
+                gen = self.trace.generation(
+                    name=name,
+                    model=model,
+                    input=input,
+                    output=output,
+                    usage=usage,
+                    metadata=metadata or {},
+                )
+                return gen
+            except Exception as e:
+                logger.warning(
+                    "langfuse_generation_failed",
+                    error=str(e),
+                    name=name,
+                    model=model,
+                )
+                return None
+        else:
+            # Langfuse v4.x: 透過 OTEL 追蹤，這裡只記錄 debug log
+            logger.debug(
+                "langfuse_generation_logged",
                 name=name,
                 model=model,
+                trace_id=self._langfuse_trace_id,
             )
             return None
 
@@ -207,13 +228,15 @@ class LangfuseTraceContext:
             name: Span 名稱
             metadata: 額外 metadata
         """
-        if not self.trace:
-            return None
-
-        try:
-            return self.trace.span(name=name, metadata=metadata or {})
-        except Exception as e:
-            logger.warning("langfuse_span_failed", error=str(e), name=name)
+        if self.trace and hasattr(self.trace, "span"):
+            try:
+                return self.trace.span(name=name, metadata=metadata or {})
+            except Exception as e:
+                logger.warning("langfuse_span_failed", error=str(e), name=name)
+                return None
+        else:
+            # Langfuse v4.x: OTEL spans
+            logger.debug("langfuse_span_logged", name=name, trace_id=self._langfuse_trace_id)
             return None
 
     def score(
@@ -230,20 +253,26 @@ class LangfuseTraceContext:
             value: 分數 (0.0 - 1.0)
             comment: 評論
         """
-        if not self.trace:
-            return
-
-        try:
-            self.trace.score(
+        if self.trace and hasattr(self.trace, "score"):
+            try:
+                self.trace.score(
+                    name=name,
+                    value=value,
+                    comment=comment,
+                )
+            except Exception as e:
+                logger.warning(
+                    "langfuse_score_failed",
+                    error=str(e),
+                    name=name,
+                )
+        else:
+            # Langfuse v4.x: score via OTEL attributes
+            logger.debug(
+                "langfuse_score_logged",
                 name=name,
                 value=value,
-                comment=comment,
-            )
-        except Exception as e:
-            logger.warning(
-                "langfuse_score_failed",
-                error=str(e),
-                name=name,
+                trace_id=self._langfuse_trace_id,
             )