feat: Python SDK real implementation + API ingestion routes

- SDK: client with BatchTransport, trace decorator/context manager,
  log_decision, thread-local context stack, nested trace→span support
- API: POST /api/traces (batch ingest), GET /api/traces (paginated list),
  GET /api/traces/[id] (full trace with relations), GET /api/health
- Tests: 8 unit tests for SDK (all passing)
- Transport: thread-safe buffer with background flush thread

apps/web/src/app/api/traces/[id]/route.ts

@@ -0,0 +1,46 @@
+import { NextResponse } from "next/server";
+import { prisma } from "@/lib/prisma";
+
+// GET /api/traces/[id] — Get single trace with all relations
+export async function GET(
+  _request: Request,
+  { params }: { params: Promise<{ id: string }> }
+) {
+  try {
+    const { id } = await params;
+
+    if (!id || typeof id !== "string") {
+      return NextResponse.json({ error: "Invalid trace ID" }, { status: 400 });
+    }
+
+    const trace = await prisma.trace.findUnique({
+      where: { id },
+      include: {
+        decisionPoints: {
+          orderBy: {
+            timestamp: "asc",
+          },
+        },
+        spans: {
+          orderBy: {
+            startedAt: "asc",
+          },
+        },
+        events: {
+          orderBy: {
+            timestamp: "asc",
+          },
+        },
+      },
+    });
+
+    if (!trace) {
+      return NextResponse.json({ error: "Trace not found" }, { status: 404 });
+    }
+
+    return NextResponse.json({ trace }, { status: 200 });
+  } catch (error) {
+    console.error("Error retrieving trace:", error);
+    return NextResponse.json({ error: "Internal server error" }, { status: 500 });
+  }
+}