feat: add multi-user RBAC with admin/viewer roles and user management

Add complete multi-user support with role-based access control:

Backend:
- Add users CRUD endpoints (GET/POST/PUT/DELETE /api/users) with admin-only guards
- Add display_name column to users table with ALTER TABLE migration
- Fix auth to use session-based user identity (not hardcoded 'admin')
- Add POST /api/auth/logout to revoke server-side sessions
- Add require_admin() and extract_session() helpers for clean RBAC
- Guard all mutating endpoints (clients, providers, models, settings, backup)

Frontend:
- Add Users management page with create/edit/reset-password/delete modals
- Add role gating: hide edit/delete buttons for viewers on clients, providers, models
- Settings page hides auth tokens and admin actions for viewers
- Logout now revokes server session before clearing localStorage
- Sidebar shows real display_name and formatted role (Administrator/Viewer)
- Fix sidebar header: single logo with onerror fallback, renamed to 'LLM Proxy'
- Add badge and btn-action CSS classes for role pills and action buttons
- Bump cache-bust to v=7

static/css/dashboard.css

@@ -840,6 +840,24 @@ body {
     border: 1px solid var(--bg3);
 }
 
+/* Role / status badges */
+.badge {
+    padding: 0.2rem 0.6rem;
+    border-radius: 9999px;
+    font-size: 0.7rem;
+    font-weight: 700;
+    text-transform: uppercase;
+    display: inline-flex;
+    align-items: center;
+    gap: 0.25rem;
+    letter-spacing: 0.03em;
+}
+
+.badge-success { background: rgba(184, 187, 38, 0.2); color: var(--green-light); }
+.badge-info { background: rgba(69, 133, 136, 0.2); color: var(--blue-light); }
+.badge-warning { background: rgba(250, 189, 47, 0.2); color: var(--yellow-light); }
+.badge-danger { background: rgba(251, 73, 52, 0.2); color: var(--red-light); }
+
 /* Charts */
 .chart-container {
     position: relative;
@@ -907,6 +925,37 @@ body {
 .btn-danger { background: var(--red); color: var(--fg0); }
 .btn-danger:hover { background: var(--red-light); }
 
+/* Small inline action buttons (edit, delete, copy) */
+.btn-action {
+    background: none;
+    border: 1px solid var(--bg3);
+    color: var(--fg3);
+    padding: 0.35rem 0.5rem;
+    border-radius: 6px;
+    cursor: pointer;
+    transition: all 0.2s;
+    display: inline-flex;
+    align-items: center;
+    justify-content: center;
+}
+
+.btn-action:hover {
+    background: var(--bg2);
+    color: var(--fg0);
+    border-color: var(--bg4);
+}
+
+.btn-action.danger:hover {
+    background: rgba(251, 73, 52, 0.15);
+    color: var(--red-light);
+    border-color: var(--red);
+}
+
+.action-buttons {
+    display: flex;
+    gap: 0.5rem;
+}
+
 /* Modals */
 .modal {
     position: fixed;