feat(api): Phase 15.2 Redis Trace Context 傳遞

實現 Redis Streams 跨服務追蹤零斷鏈:
- telemetry.py: 新增 get_trace_context() + restore_trace_context()
- webhooks.py: Producer 注入 _trace_id, _span_id 到 Redis
- signal_worker.py: Consumer 還原 Trace Context 建立子 Span

架構: API → Redis Streams → Worker 完整追蹤鏈
格式: W3C Trace Context (traceparent)

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

apps/api/src/core/telemetry.py

@@ -219,3 +219,121 @@ def get_current_trace_id() -> str | None:
         return None
 
     return format(ctx.trace_id, '032x')
+
+
+def get_current_span_id() -> str | None:
+    """
+    Get current span ID
+
+    Returns:
+        Span ID as hex string, or None if no active span
+    """
+    span = trace.get_current_span()
+    if span is None:
+        return None
+
+    ctx = span.get_span_context()
+    if ctx is None or not ctx.is_valid:
+        return None
+
+    return format(ctx.span_id, '016x')
+
+
+# =============================================================================
+# Phase 15.2: Redis Trace Context Propagation
+# =============================================================================
+
+def get_trace_context() -> dict[str, str] | None:
+    """
+    取得當前 Trace Context 用於 Redis Streams 注入
+
+    Phase 15.2: 解決 Redis Streams Trace 斷鏈問題
+
+    Returns:
+        dict with trace_id, span_id, or None if no active span
+
+    Usage (寫入 Redis):
+        payload = {**signal.dict(), "_trace_context": get_trace_context()}
+        await redis.xadd("stream:signals", payload)
+    """
+    trace_id = get_current_trace_id()
+    span_id = get_current_span_id()
+
+    if not trace_id:
+        return None
+
+    return {
+        "trace_id": trace_id,
+        "span_id": span_id or "",
+    }
+
+
+def restore_trace_context(trace_context: dict[str, str] | None):
+    """
+    從 Redis 訊息還原 Trace Context 並建立新 Span
+
+    Phase 15.2: Worker 端 Context 重建
+
+    Args:
+        trace_context: 從 Redis 訊息取得的 _trace_context
+
+    Returns:
+        Context manager for the restored span
+
+    Usage (讀取 Redis):
+        message = await redis.xreadgroup(...)
+        trace_ctx = message.get("_trace_context")
+
+        with restore_trace_context(trace_ctx) as span:
+            # 處理邏輯，此處的 span 會繼承原始 trace_id
+            pass
+    """
+    from contextlib import contextmanager
+
+    from opentelemetry.trace import SpanKind
+    from opentelemetry.trace.propagation.tracecontext import TraceContextTextMapPropagator
+
+    tracer = get_tracer("awoooi.worker")
+
+    @contextmanager
+    def _context_manager():
+        if not trace_context or not trace_context.get("trace_id"):
+            # 沒有 trace context，建立新的 span
+            with tracer.start_as_current_span(
+                "worker_process",
+                kind=SpanKind.CONSUMER,
+            ) as span:
+                yield span
+            return
+
+        # 有 trace context，嘗試還原
+        try:
+            # 使用 W3C Trace Context 格式建立 carrier
+            carrier = {
+                "traceparent": f"00-{trace_context['trace_id']}-{trace_context.get('span_id', '0' * 16)}-01"
+            }
+
+            # 從 carrier 提取 context
+            propagator = TraceContextTextMapPropagator()
+            ctx = propagator.extract(carrier=carrier)
+
+            # 建立子 span 繼承原始 trace
+            with tracer.start_as_current_span(
+                "worker_process",
+                context=ctx,
+                kind=SpanKind.CONSUMER,
+            ) as span:
+                span.set_attribute("trace.restored", True)
+                span.set_attribute("trace.parent_trace_id", trace_context["trace_id"])
+                yield span
+
+        except Exception as e:
+            _logger.warning(f"Trace context restore failed: {e}, creating new span")
+            with tracer.start_as_current_span(
+                "worker_process",
+                kind=SpanKind.CONSUMER,
+            ) as span:
+                span.set_attribute("trace.restore_failed", str(e))
+                yield span
+
+    return _context_manager()