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/routes/messages.py

@@ -26,6 +26,7 @@ from app.models import Message, MessageKey, User
 from app.ratelimit import RateLimiter
 from app.realtime import manager
 from app.security import verify_password
+from app.userscope import is_messageable_user, messageable_clauses
 
 router = APIRouter(prefix="/api/messages", tags=["messages"])
 
@@ -101,18 +102,6 @@ def _serialize_msg(m: Message) -> dict:
     }
 
 
-def _messageable() -> list:
-    """SQL clauses for "a real, active, non-suspended human" — for directory/recipient queries."""
-    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()` 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)
 
 
 def ensure_welcome(db: Session, user: User) -> None:
@@ -221,7 +210,21 @@ def reset_keys(
 def public_key(
     user_id: int, user: User = Depends(require_human), db: Session = Depends(get_db)
 ) -> dict:
-    """A member's public key, for deriving the pairwise conversation key."""
+    """A member's public key, for deriving the pairwise conversation key. MB2 (like get_thread):
+    resolve the target only if they're messageable OR we already share a thread, else the same
+    "User not found" as a non-existent id — so this can't be probed to enumerate user_ids or to
+    fetch keys for suspended/deactivated/demo accounts."""
+    cond = and_(
+        Message.kind == "user",
+        or_(
+            and_(Message.sender_id == user.id, Message.recipient_id == user_id),
+            and_(Message.sender_id == user_id, Message.recipient_id == user.id),
+        ),
+    )
+    p = db.get(User, user_id)
+    has_history = db.scalar(select(Message.id).where(cond).limit(1))
+    if p is None or (not is_messageable_user(p) and not has_history):
+        raise HTTPException(status_code=404, detail="User not found")
     k = db.get(MessageKey, user_id)
     if k is None:
         raise HTTPException(status_code=404, detail="That member hasn't set up messaging yet.")
@@ -240,7 +243,7 @@ def list_recipients(
         db.execute(
             select(User)
             .join(MessageKey, MessageKey.user_id == User.id)
-            .where(User.id != user.id, *_messageable())
+            .where(User.id != user.id, *messageable_clauses())
             .order_by(func.lower(func.coalesce(User.display_name, User.email)))
         )
         .scalars()