Added LINE hook for integration

[chr-nguyen]

Justin's notifier runs on a schedule, checks the member RSS feeds, and when it finds a new post it sends a POST request to whatever URL is set in COMMUNITY_FEED_GENERIC_WEBHOOK_URL — which we'll point at our LINE hook. The body it sends looks like this:

  {                                                                                                                  
    "event": "community_feed_item",
    "item": { ... },
    "message": "New community post from Ash Ryan Arnwine\nSome Blog Post Title\nhttps://example.com/post"
  }  

The message field is the plain text string built by buildMessage() — author name, post title, and link joined by newlines.

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Our LINE hook receives that request and does three things:

1. Checks the secret — if the x-webhook-secret header doesn't match, it rejects the request immediately
2. Pulls out body.message — ignores everything else in the payload, just takes that plain text string
3. Forwards it to LINE — POSTs to the LINE Messaging API with the message as a plain text message, delivered to whatever group or user LINE_TARGET_ID points to

So the LINE message your members will see is just:

New community post from Ash Ryan Arnwine
Some Blog Post Title
https://example.com/post

What changed

3 files modified, 2 files created:

• astro.config.mjs — switched to hybrid output mode and added the Cloudflare adapter so the site can serve server-side routes alongside its static pages
• src/pages/api/line-hook.ts (new) — the webhook endpoint that authenticates incoming requests and forwards the message payload to LINE's Messaging API
• scripts/community-feed-notifier.mjs — added x-webhook-secret header to Justin's generic webhook sender so it can authenticate against the LINE hook
• .github/workflows/community-feed-notifier.yml — passes the new COMMUNITY_FEED_GENERIC_WEBHOOK_SECRET secret into the notifier job
• .env.example (new) — documents all required env vars for both sides of the integration

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Next steps

1. Get your LINE credentials — create a Messaging API channel at developers.line.biz, issue a channel access token and grab the target group/user ID
2. Set secrets in Cloudflare Pages — WEBHOOK_SECRET, LINE_CHANNEL_TOKEN, LINE_TARGET_ID
3. Set secrets in GitHub Actions — COMMUNITY_FEED_GENERIC_WEBHOOK_URL (pointing to https://kyototechmeetup.com/api/line-hook) and COMMUNITY_FEED_GENERIC_WEBHOOK_SECRET (same value as WEBHOOK_SECRET)
4. Test end-to-end — trigger the notifier manually via workflow_dispatch with dry_run: false and verify a message lands in LINE