fix: use full IP for userId to prevent quota collision (#400)

* fix: use full IP for userId to prevent quota collision

- Remove .slice(0, 8) from base64 encoded IP
- Each IP now has unique userId (no /16 collision)
- Affects: quota tracking, Langfuse tracing

* refactor: extract getUserIdFromRequest to shared utility

- Create lib/user-id.ts with shared function
- Fix misleading 'privacy' comment (base64 is not privacy)
- Remove duplicate code from chat and log-feedback routes

lib/user-id.ts

@@ -0,0 +1,12 @@
+/**
+ * Generate a userId from request for tracking purposes.
+ * Uses base64url encoding of IP for URL-safe identifier.
+ * Note: base64 is reversible - this is NOT privacy protection.
+ */
+export function getUserIdFromRequest(req: Request): string {
+    const forwardedFor = req.headers.get("x-forwarded-for")
+    const rawIp = forwardedFor?.split(",")[0]?.trim() || "anonymous"
+    return rawIp === "anonymous"
+        ? rawIp
+        : `user-${Buffer.from(rawIp).toString("base64url")}`
+}