feat(security): R4 S3 — abuse & leak hardening

- C-3.7: fold a per-link password_version into the signed watch grant (migration
  0059 adds the column) and bump it on every password change, so rotating a share
  link's password invalidates outstanding grants at once instead of letting them
  live out the 6h TTL. make_grant/check_grant take the version; update_link bumps.
- C-3.8: the share recipient picker and share-by-email now reuse the messageable
  filter (not-demo AND active AND not-suspended) instead of only excluding demo,
  so suspended/deactivated addresses aren't leaked or reachable as targets.
- C-3.9: GET /keys/{user_id} adopts get_thread's MB2 guard — a non-messageable
  target with no shared history returns the same "User not found" as a missing id,
  so it can't be probed to enumerate users or fetch suspended users' keys.
- C-3.12: _register_account catches IntegrityError on the insert (the select-then-
  insert TOCTOU) and treats it as the already-registered no-op.
- C-3.13: trigger_job claims the running flag atomically under _activity_lock
  (compare-and-set) instead of a bare check, closing the double-start window.
- Move _messageable/is_messageable_user into a shared app/userscope.py so downloads
  doesn't import the messages route module.

backend/app/userscope.py

@@ -0,0 +1,22 @@
+"""Shared "who is a real, active, non-suspended human" predicate.
+
+Used wherever a user must be filtered out of a directory / recipient list or rejected as a target:
+the messaging directory AND the download share-by-email / recipient-picker endpoints. Kept in one
+place so the two forms (SQL clauses for a query, Python check for a loaded row) can't drift apart,
+and so a route module doesn't have to import another route module just to reuse the rule.
+"""
+from app.models import User
+
+
+def messageable_clauses() -> list:
+    """SQL clauses for "a real, active, non-suspended human" — splat into a directory/recipient query."""
+    return [
+        User.is_demo.is_(False),
+        User.is_active.is_(True),
+        User.is_suspended.is_(False),
+    ]
+
+
+def is_messageable_user(u: User | None) -> bool:
+    """Python mirror of `messageable_clauses()` for an already-loaded row (WS auth, send recipient)."""
+    return bool(u and not u.is_demo and u.is_active and not u.is_suspended)