fix: remove webmentions-proxy, fix sender timing, explicit mountPath

- Remove @rmdes/indiekit-endpoint-webmentions-proxy (redundant with
  webmention-io which already provides a public JSON API from MongoDB)
- Remove proxy-related variables (webmentionsProxyMountPath, cacheTtl)
- Add explicit mountPath to webmention-io config to avoid future surprises
- Increase WEBMENTION_SENDER_POLL_INTERVAL to 600s so deploys complete
  before the poller fires, preventing silent data loss where posts with
  no stored content get marked webmention-sent before the page is live

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

indiekit.config.mjs

@@ -125,15 +125,8 @@ const webmentionSenderTimeout = Number.isFinite(webmentionSenderTimeoutRaw)
   : 10000;
 const webmentionSenderUserAgent =
   process.env.WEBMENTION_SENDER_USER_AGENT || `${siteName} Webmention Sender`;
-const webmentionsProxyMountPath =
+const webmentionIoMountPath =
-  process.env.WEBMENTIONS_PROXY_MOUNT_PATH || "/webmentions-api";
+  process.env.WEBMENTION_IO_MOUNT_PATH || "/webmentions";
-const webmentionsProxyCacheTtlRaw = Number.parseInt(
-  process.env.WEBMENTIONS_PROXY_CACHE_TTL || "60",
-  10,
-);
-const webmentionsProxyCacheTtl = Number.isFinite(webmentionsProxyCacheTtlRaw)
-  ? webmentionsProxyCacheTtlRaw
-  : 60;
 const commentsMountPath = process.env.COMMENTS_MOUNT_PATH || "/comments";
 const commentsRateLimitPerHourRaw = Number.parseInt(
   process.env.COMMENTS_RATE_LIMIT_PER_HOUR || "5",
@@ -301,7 +294,6 @@ export default {
     "@rmdes/indiekit-preset-eleventy",
     "@rmdes/indiekit-endpoint-github",
     "@rmdes/indiekit-endpoint-webmention-io",
-    "@rmdes/indiekit-endpoint-webmentions-proxy",
     "@rmdes/indiekit-endpoint-webmention-sender",
     "@rmdes/indiekit-endpoint-homepage",
     "@rmdes/indiekit-endpoint-conversations",
@@ -350,15 +342,10 @@ export default {
     username: githubUsername,
   },
   "@rmdes/indiekit-endpoint-webmention-io": {
+    mountPath: webmentionIoMountPath,
     token: process.env.WEBMENTION_IO_TOKEN,
     domain: webmentionDomain,
   },
-  "@rmdes/indiekit-endpoint-webmentions-proxy": {
-    mountPath: webmentionsProxyMountPath,
-    token: process.env.WEBMENTION_IO_TOKEN,
-    domain: webmentionDomain,
-    cacheTtl: webmentionsProxyCacheTtl,
-  },
   "@rmdes/indiekit-endpoint-webmention-sender": {
     mountPath: webmentionSenderMountPath,
     timeout: webmentionSenderTimeout,