feat(server): port the UO models, utils and schema fragment

The data half of the extraction: 8 model directories, 13 utils, the shard
stream catalog and the 27-table schema fragment with its purge.

server/core.js is what makes the port a one-line import change per file rather
than a signature change per function. Ported code requires its dependencies at
file scope -- `const { query } = require('../../core')` -- which runs before
register() has been called and before any ctx exists. So every member is a
stable function that resolves ctx when CALLED, and nothing may be destructured
off ctx at init either, because core is free to hand over a getter.

Two helpers are vendored rather than taken from ctx, and the line between them
is the point. utils/excerpt.js is core's deriveExcerpt -- nine lines of pure
text handling. Core's sanitiser next to it was NOT copied: a second copy of a
security control diverges silently the moment either is fixed. announceLinks.js
vendors legError and articleUrl the same way, but baseUrl could not be: core's
reads APP_BASE_URL, and §2.7 forbids a module reading core's environment, so it
comes off ctx.site.baseUrl.

The schema fragment is core's 27 shard_*/uo_link_* statements, verbs CREATE,
ALTER and UPDATE only, every CREATE TABLE guarded. Two of its tables carry a
foreign key INTO users, which is allowed and is why the replay order matters --
core's schema is in place before this runs. The reverse never occurs and must
not: it would make core unable to boot without a module installed.

One real port bug caught by the integration run, not by tests: the atlas art
map resolved `../../../db/data`, which pointed at core's tree when this file
lived there and points outside server/ now. A path that happens to resolve is
exactly what survives a green suite, because the absent-file branch returns {}
and looks like the normal case.

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

server/model/shardLinks/shardLinks.db.js

@@ -0,0 +1,42 @@
+const { query } = require('../../core')
+
+const COLS = 'account, user_id, char_name, linked_at'
+
+// Upsert a link. account is the PK, so a re-link moves the account to the new
+// user (the sidecar already treats /link/confirm as authoritative).
+async function upsert({ account, userId, charName }) {
+  await query(
+    `INSERT INTO shard_account_links (account, user_id, char_name)
+     VALUES (?, ?, ?)
+     ON DUPLICATE KEY UPDATE user_id = VALUES(user_id), char_name = VALUES(char_name)`,
+    [account, userId, charName || null],
+  )
+  return getByAccount(account)
+}
+
+async function getByAccount(account) {
+  const rows = await query(`SELECT ${COLS} FROM shard_account_links WHERE account = ? LIMIT 1`, [account])
+  return rows[0] || null
+}
+
+const listByUser = (userId) =>
+  query(`SELECT ${COLS} FROM shard_account_links WHERE user_id = ? ORDER BY linked_at DESC`, [userId])
+
+async function isOwnedBy(account, userId) {
+  const rows = await query(
+    'SELECT 1 FROM shard_account_links WHERE account = ? AND user_id = ? LIMIT 1',
+    [account, userId],
+  )
+  return rows.length > 0
+}
+
+const remove = (account, userId) =>
+  query('DELETE FROM shard_account_links WHERE account = ? AND user_id = ?', [account, userId])
+
+// Drop the mirror for an account regardless of which user held it — used to
+// reconcile when the tie is severed at the source (an in-game [unlink →
+// account.unlinked event, or a site-side DELETE /link/{account}).
+const removeByAccount = (account) =>
+  query('DELETE FROM shard_account_links WHERE account = ?', [account])
+
+module.exports = { upsert, getByAccount, listByUser, isOwnedBy, remove, removeByAccount }