feat(teams): the forum schema, the operator's two switches, and the ack gate

The whole forum schema lands at once — threads, posts, the moderation ledger and
upload attribution — including the columns only phase 5's discussion threads use.
That is TEAMS.md 5.1's split BY LAYER rather than by feature: phase 5 opens paths
instead of migrating data.

Three settings keys, and only one of them is ordinary. `teams_forums_enabled` and
`teams_forum_images` are enum keys on the existing admin settings endpoint;
`teams_forum_images` also carries a server-side PRECONDITION, which is why the
three live in their own model rather than in the generic setMany() loop where a
reader would never find it.

The gate is the server's. `PUT teams_forum_images = 'uploads'` is rejected 400
unless the same request carries the acknowledgement version — the admin checkbox
is how the gate is presented, never the gate. What is stored is the TEXT VERSION,
so "which wording did they agree to" is answerable later; settings already record
updated_by/updated_at, and an activity_log row puts it in the staff audit trail.

A reworded notice makes a stored acknowledgement stale, and neither obvious answer
is right: uploads KEEP WORKING, and no other forum setting may be saved until it is
re-given. Non-destructive, and impossible to ignore.

Both reads fail closed. A DB fault reports the forum off and images disabled — a
forum that 404s for a minute is the cheap failure; a policy that is not a policy
is not.

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

server/src/model/settings/settings.db.js

@@ -17,6 +17,20 @@ async function set(key, value, updatedBy = null) {
   )
 }
 
+// One row WITH its provenance. `updated_by`/`updated_at` are already stored for
+// every key; this is the only reader that needs them, because TEAMS.md §5.5.5
+// makes the uploads acknowledgement a RECORDED consent rather than a displayed
+// one, and "which admin accepted it, and when" is the question that has to be
+// answerable afterwards.
+async function getRow(key) {
+  const rows = await query(
+    'SELECT s.`key`, s.value, s.updated_by, s.updated_at, u.username AS updated_by_username '
+      + 'FROM settings s LEFT JOIN users u ON u.id = s.updated_by WHERE s.`key` = ? LIMIT 1',
+    [key],
+  )
+  return rows[0] || null
+}
+
 // Insert a default only if the key does not already exist.
 async function seedDefault(key, value) {
   await query('INSERT IGNORE INTO settings (`key`, value) VALUES (?, ?)', [key, value])
@@ -30,4 +44,4 @@ async function remove(key) {
   await query('DELETE FROM settings WHERE `key` = ?', [key])
 }
 
-module.exports = { getAll, get, set, seedDefault, remove }
+module.exports = { getAll, get, getRow, set, seedDefault, remove }