Let session owners reassign their session to another user

Previously a session could only be reassigned to another user via a
manual DB UPDATE. This adds it to the session edit UI for every owner of
a session (not just admins) - the ownership check in HandleUpdate
(existing.UserId == userId) already guarantees the caller owns the
session, so handing over one's own session is safe.

Backend:
- ProfileEndpoint: new GET /api/users returning a minimal user directory
  (userId/username/displayName only) for any logged-in user. The
  admin-gated /api/admin/users stays the source for sensitive fields
  (defaultPath/canUseCli/isAdmin).
- SessionEndpoint.HandleUpdate: validate the target user exists (400 if
  not) and pass allowUserReassign: true to UpdateSession.
- DatabaseService.UpdateSession: new allowUserReassign flag; only then is
  a non-empty userId applied as user_id (empty/null is ignored - a
  session must always have an owner).

Frontend:
- index.html: hidden #sessionOwnerRow with a user picker card in the
  session edit form (existing sessions only).
- cards.js: buildUserCard helper for the picker.
- session-page.js: initSessionUserPicker loads users via apiGetUsers(),
  preselects the current owner, and on an actual owner change asks for
  confirmation (irreversible - the session leaves the user's own list).
- api.js: apiGetUsers() -> /api/users.
- ARCHITECTURE.md: document /api/users.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

wwwroot/index.html

@@ -174,6 +174,14 @@
               <button type="button" id="sessionTelegramUnlink" class="icon-btn danger-btn" title="Verknüpfung lösen"><i class="fa fa-chain-broken"></i></button>
             </div>
             <small class="form-hint">Wird automatisch gesetzt wenn jemand /start an den Bot sendet.</small>
+            <div id="sessionOwnerRow" hidden>
+              <label>Zugewiesener Benutzer</label>
+              <div id="sessionUserSelectCard" class="agent-select-card">
+                <div class="agent-select-empty"><i class="fa fa-user"></i> Kein Benutzer</div>
+              </div>
+              <input type="hidden" id="sessionUserSelect" value="">
+              <small class="form-hint">Weist diese Session einem anderen Benutzer zu. Nach dem Speichern verschwindet sie aus deiner Übersicht.</small>
+            </div>
           </div>
         </div>
         <div class="form-actions">