feat: client and server logic to manage announcements (#3)

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### Attestations

- [x] I have read and agree to the [Code of Conduct](https://docs.nhcarrigan.com/community/coc/)
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- [x] I have pinned the dependencies to a specific patch version.

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- [x] I have run the linter and resolved any errors.
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### Tests

- [ ] My contribution adds new code, and I have added tests to cover it.
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Reviewed-on: #3
Co-authored-by: Naomi Carrigan <commits@nhcarrigan.com>
Co-committed-by: Naomi Carrigan <commits@nhcarrigan.com>

server/src/hooks/ips.ts

@@ -0,0 +1,36 @@
+/**
+ * @copyright nhcarrigan
+ * @license Naomi's Public License
+ * @author Naomi Carrigan
+ */
+
+import { blockedIps } from "../cache/blockedIps.js";
+import { getIpFromRequest } from "../modules/getIpFromRequest.js";
+import type { onRequestHookHandler } from "fastify";
+
+/**
+ * Ensures that form submissions only come from our web application.
+ * @param request - The request payload from the server.
+ * @param response - The reply handler from Fastify.
+ * @returns A Fastify reply if the request is invalid, otherwise undefined.
+ */
+// eslint-disable-next-line @typescript-eslint/no-misused-promises -- For reasons I cannot comprehend, Fastify seems to require us to return a request?
+export const ipHook: onRequestHookHandler = async(request, response) => {
+  const ip = getIpFromRequest(request);
+  const ipRecord = blockedIps.find(
+    (record) => {
+      return record.ip === ip && record.ttl > new Date();
+    },
+  );
+  if (ipRecord && ipRecord.ttl > new Date()) {
+    return await response.
+      status(403).
+      send({
+        error: `Your IP address (${ipRecord.ip}) has been blocked until ${ipRecord.ttl.toISOString()}, to protect our API against brute-force attacks.`,
+      });
+  }
+  if (ipRecord && ipRecord.ttl <= new Date()) {
+    blockedIps.splice(blockedIps.indexOf(ipRecord), 1);
+  }
+  return undefined;
+};