feat: per-role inject_mode, OTR fixes, hover metadata, send/stop tooltip

- inject_mode: per-role toggle (parallel to inject_datetime) gates the
  "Current mode: Off The Record" line in the system prompt; wired through
  model_registry, context_loader, chat router, orchestrator router, and
  local_llm settings UI

- OTR orchestrator fix: OrchestrateRequest now carries off_record;
  _finalize_job stores it per message and gates log_turn on it; JS
  orchestrate payload sends off_record correctly

- Per-message hover metadata: removed always-visible .model-tag; replaced
  with .msg-meta strip in the action bar (hover-only); shows model label,
  host, fallback indicator, and OTR badge; stored in session JSON

- Send/stop button tooltip: shows role + model and (when tools on)
  separate orchestrator model + engine label; live elapsed timer on stop
  button via startRunTimer/stopRunTimer

- OrchestratorResult.backend_label: new field; openai_orchestrator fills
  it; finalize_job propagates it to job dict and session messages

- GET /backend: exposes orchestrator_model label so the frontend tooltip
  can show both models separately

- TODO: session delete confirmation added

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

cortex/static/style.css

@@ -614,18 +614,34 @@
         .copy-btn:hover  { color: var(--text); border-color: var(--muted); }
         .copy-btn.copied { color: var(--success); border-color: var(--success-dim); }
 
-        /* Model tag — shown at the bottom of every assistant message */
-        .model-tag {
-            display: block;
-            font-size: 0.67rem;
-            color: #475569;
-            margin-top: 0.55rem;
-            padding-top: 0.4rem;
-            border-top: 1px solid #2d3148;
-            text-align: right;
+        /* Message metadata — shown in the hover bar below the bubble */
+        .msg-meta {
+            display: flex;
+            align-items: center;
+            gap: 5px;
+            flex: 1;
+            min-width: 0;
+            font-size: 0.62rem;
+            color: var(--dim);
             letter-spacing: 0.02em;
+            overflow: hidden;
         }
-        .model-tag.fallback { color: #f59e0b; }
+        .msg-meta-model {
+            overflow: hidden;
+            text-overflow: ellipsis;
+            white-space: nowrap;
+        }
+        .msg-meta-model.fallback { color: #f59e0b; }
+        .msg-meta-badge {
+            flex-shrink: 0;
+            padding: 1px 5px;
+            border-radius: 3px;
+            font-size: 0.6rem;
+            font-weight: 600;
+            letter-spacing: 0.04em;
+        }
+        .msg-meta-badge.otr { background: #1e1b4b; color: #818cf8; }
+        [data-theme="light"] .msg-meta-badge.otr { background: #ede9fe; color: #5b21b6; }
 
         /* Retry button — shown in error message bubbles */
         .retry-btn {