fix(llmops): 鎖定 langfuse SDK v2.x (v3.x/v4.x 均已移除 client.trace() API)

排查確認：v3.x 也無 client.trace()，langfuse_client.py 依賴此 API。
鎖定 <3.0.0 確保安裝 v2.60.10 (v2 最新)，trace/generation/score 均可用。

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

apps/api/pyproject.toml

@@ -27,8 +27,9 @@ dependencies = [
     # Sentry (Error Tracking - 補強 SignOz，Self-Hosted @ 192.168.0.110)
     "sentry-sdk[fastapi]>=2.0.0",
     # Langfuse (LLMOps - Phase 15.1，Self-Hosted @ 192.168.0.110:3100)
-    # 2026-04-01 Claude Code: 鎖定 v2.x — v4.x 改用 OTLP，與 langfuse_client.py 的 client.trace() 不相容
-    "langfuse>=2.0.0,<4.0.0",
+    # 2026-04-02 Claude Code: 鎖定 v2.60.x — v3.x/v4.x 移除 client.trace() API，與 langfuse_client.py 不相容
+    # v2.x 最高版 = 2.60.10，有 client.trace()、trace.generation()、trace.score()
+    "langfuse>=2.0.0,<3.0.0",
     # Prometheus (ADR-037 Wave A.5 監控指標)
     "prometheus-client>=0.20.0",
     # Phase 6.4g: leWOOOgo Brain - 積木化決策引擎

apps/api/requirements.txt

@@ -40,8 +40,8 @@ opentelemetry-instrumentation-logging>=0.41b0
 # Langfuse (LLMOps Observability)
 # Phase 15.1: LLM 呼叫追蹤、成本監控、Prompt 版本管理
 # ==========================================================================
-# 2026-04-01 Claude Code: 鎖定 v2.x — v4.x 改用 OTLP，與 langfuse_client.py 的 client.trace() 不相容
-langfuse>=2.0.0,<4.0.0
+# 2026-04-02 Claude Code: 鎖定 v2.60.x — v3.x/v4.x 移除 client.trace() API，與 langfuse_client.py 不相容
+langfuse>=2.0.0,<3.0.0
 
 # Development
 pytest>=7.4.0