Bsky short link service (#4542)

* bskylink: scaffold service w/ initial config and schema

* bskylink: implement link creation and redirects

* bskylink: tidy

* bskylink: tests

* bskylink: tidy, add error handler

* bskylink: add dockerfile

* bskylink: add build

* bskylink: fix some express plumbing

* bskyweb: proxy fallthrough routes to link service redirects

* bskyweb: build w/ link proxy

* Add AASA to bskylink (#4588)

---------

Co-authored-by: Hailey <me@haileyok.com>

bskylink/src/routes/redirect.ts

@@ -0,0 +1,40 @@
+import assert from 'node:assert'
+
+import {DAY, SECOND} from '@atproto/common'
+import {Express} from 'express'
+
+import {AppContext} from '../context.js'
+import {handler} from './util.js'
+
+export default function (ctx: AppContext, app: Express) {
+  return app.get(
+    '/:linkId',
+    handler(async (req, res) => {
+      const linkId = req.params.linkId
+      assert(
+        typeof linkId === 'string',
+        'express guarantees id parameter is a string',
+      )
+      const found = await ctx.db.db
+        .selectFrom('link')
+        .selectAll()
+        .where('id', '=', linkId)
+        .executeTakeFirst()
+      if (!found) {
+        // potentially broken or mistyped link— send user to the app
+        res.setHeader('Location', `https://${ctx.cfg.service.appHostname}`)
+        res.setHeader('Cache-Control', 'no-store')
+        return res.status(302).end()
+      }
+      // build url from original url in order to preserve query params
+      const url = new URL(
+        req.originalUrl,
+        `https://${ctx.cfg.service.appHostname}`,
+      )
+      url.pathname = found.path
+      res.setHeader('Location', url.href)
+      res.setHeader('Cache-Control', `max-age=${(7 * DAY) / SECOND}`)
+      return res.status(301).end()
+    }),
+  )
+}