Member invites, gallery resync, content and operator docs

- Invite flow: admin issue/revoke on join requests, public single-use
  claim links (hash-only tokens, 7-day expiry, no state reveal), claim
  creates the Member account and marks the request accepted
- Gallery: admin status page plus resync endpoint; sync refreshes
  thumbnails whose content changed; tools/gallery contract and checker
- Docs: public content page, admin-only operator runbook, api.md
  invite/gallery sections, all routes in the live API docs, docs
  reachability gate test
- Screenshots cover the new pages; version 1.0.18

tests/api/people.py

@@ -1,8 +1,12 @@
 # retoor <retoor@molodetz.nl>
+import re
+
+USERNAME_PATTERN = re.compile(r"^[A-Za-z0-9_-]{3,32}$")
 
 
 def test_people_lists_real_accounts_only(anon):
     people = anon.get("/people", headers={"Accept": "application/json"}).json()
     names = [person["username"] for person in people.get("people", [])]
     assert names[0] == "retoor"
-    assert set(names) <= {"retoor", "lezer"}
+    assert {"retoor", "lezer"} <= set(names)
+    assert all(USERNAME_PATTERN.match(name) for name in names)