feat: implement usage tracking for AI requests

Adds local token usage tracking for all AI providers. Usage data is
captured during stream processing and stored in a local database.
Includes a new Usage tab in the Providers dashboard to visualize
historical token consumption.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

src/app/api/usage/history/route.js

@@ -0,0 +1,12 @@
+import { NextResponse } from "next/server";
+import { getUsageStats } from "@/lib/usageDb";
+
+export async function GET() {
+  try {
+    const stats = await getUsageStats();
+    return NextResponse.json(stats);
+  } catch (error) {
+    console.error("Error fetching usage stats:", error);
+    return NextResponse.json({ error: "Failed to fetch usage stats" }, { status: 500 });
+  }
+}