feat(usage): implement cost tracking backend and pricing configuration

- Add pricing constants with default rates for all providers
- Update localDb to support pricing configuration schema
- Add cost calculation logic to usageDb
- Add pricing management API endpoints
- Fix provider alias mapping for accurate cost lookups

🤖 Generated with [Claude Code](https://claude.com/claude-code)
Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

src/app/api/pricing/route.js

@@ -0,0 +1,134 @@
+import { NextResponse } from "next/server";
+import { getPricing, updatePricing, resetPricing, resetAllPricing } from "@/lib/localDb.js";
+import { getDefaultPricing } from "@/shared/constants/pricing.js";
+
+/**
+ * GET /api/pricing
+ * Get current pricing configuration (merged user + defaults)
+ */
+export async function GET() {
+  try {
+    const pricing = await getPricing();
+    return NextResponse.json(pricing);
+  } catch (error) {
+    console.error("Error fetching pricing:", error);
+    return NextResponse.json(
+      { error: "Failed to fetch pricing" },
+      { status: 500 }
+    );
+  }
+}
+
+/**
+ * PATCH /api/pricing
+ * Update pricing configuration
+ * Body: { provider: { model: { input: number, output: number, cached: number, ... } } }
+ */
+export async function PATCH(request) {
+  try {
+    const body = await request.json();
+
+    // Validate body structure
+    if (typeof body !== "object" || body === null) {
+      return NextResponse.json(
+        { error: "Invalid pricing data format" },
+        { status: 400 }
+      );
+    }
+
+    // Validate pricing structure
+    for (const [provider, models] of Object.entries(body)) {
+      if (typeof models !== "object" || models === null) {
+        return NextResponse.json(
+          { error: `Invalid pricing for provider: ${provider}` },
+          { status: 400 }
+        );
+      }
+
+      for (const [model, pricing] of Object.entries(models)) {
+        if (typeof pricing !== "object" || pricing === null) {
+          return NextResponse.json(
+            { error: `Invalid pricing for model: ${provider}/${model}` },
+            { status: 400 }
+          );
+        }
+
+        // Validate pricing fields
+        const validFields = ["input", "output", "cached", "reasoning", "cache_creation"];
+        for (const [key, value] of Object.entries(pricing)) {
+          if (!validFields.includes(key)) {
+            return NextResponse.json(
+              { error: `Invalid pricing field: ${key} for ${provider}/${model}` },
+              { status: 400 }
+            );
+          }
+          if (typeof value !== "number" || isNaN(value) || value < 0) {
+            return NextResponse.json(
+              { error: `Invalid pricing value for ${key} in ${provider}/${model}: must be non-negative number` },
+              { status: 400 }
+            );
+          }
+        }
+      }
+    }
+
+    const updatedPricing = await updatePricing(body);
+    return NextResponse.json(updatedPricing);
+  } catch (error) {
+    console.error("Error updating pricing:", error);
+    return NextResponse.json(
+      { error: "Failed to update pricing" },
+      { status: 500 }
+    );
+  }
+}
+
+/**
+ * DELETE /api/pricing
+ * Reset pricing to defaults
+ * Query params: ?provider=xxx&model=yyy (optional)
+ */
+export async function DELETE(request) {
+  try {
+    const { searchParams } = new URL(request.url);
+    const provider = searchParams.get("provider");
+    const model = searchParams.get("model");
+
+    if (provider && model) {
+      // Reset specific model
+      await resetPricing(provider, model);
+    } else if (provider) {
+      // Reset entire provider
+      await resetPricing(provider);
+    } else {
+      // Reset all pricing
+      await resetAllPricing();
+    }
+
+    const pricing = await getPricing();
+    return NextResponse.json(pricing);
+  } catch (error) {
+    console.error("Error resetting pricing:", error);
+    return NextResponse.json(
+      { error: "Failed to reset pricing" },
+      { status: 500 }
+    );
+  }
+}
+
+/**
+ * GET /api/pricing/defaults
+ * Get default pricing configuration
+ */
+export async function GET_DEFAULTS() {
+  try {
+    const defaultPricing = getDefaultPricing();
+    return NextResponse.json(defaultPricing);
+  } catch (error) {
+    console.error("Error fetching default pricing:", error);
+    return NextResponse.json(
+      { error: "Failed to fetch default pricing" },
+      { status: 500 }
+    );
+  }
+}