feat(webui): AI-provider connections and evidence-backed insights (Closes #650)

Add Phase 4 advisory surfaces for declared AI-provider connection status
(no secrets, no live probe claims) and operational insights derived only
from durable traffic, health, provider, and analytics evidence.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

webui/nav.py

@@ -4,11 +4,11 @@ Single source of truth for the console navigation so ``webui/layout.py`` and
 the ``webui/app.py`` route table stay aligned with epic #631. Read-only: every
 destination is a GET view or a Phase 1 placeholder. No mutation links.
 
-Nav groups follow the #631 Phase 1 information architecture: Health, Traffic,
+Nav groups follow the #631 information architecture: Health, Traffic,
 Runtime/Sessions, Projects, Inventory, Timeline, Policy (placeholder), and
-Insights (placeholder). Later-phase surfaces are declared as ``stub`` items and
-backed by ``STUB_PAGES`` so their nav links resolve to a graceful placeholder
-instead of a 404.
+Insights (Phase 4 providers + evidence-backed insights via #650). Later-phase
+surfaces are declared as ``stub`` items and backed by ``STUB_PAGES`` so their
+nav links resolve to a graceful placeholder instead of a 404.
 """
 
 from __future__ import annotations
@@ -65,7 +65,8 @@ NAV_GROUPS: tuple[NavGroup, ...] = (
         NavItem("/prompts", "Prompts"),
     )),
     NavGroup("Insights", (
-        NavItem("/insights", "Insights", "stub"),
+        NavItem("/insights", "Insights"),
+        NavItem("/providers", "Providers"),
         NavItem("/analytics", "Analytics"),
         NavItem("/audit", "Audit"),
     )),
@@ -89,11 +90,6 @@ STUB_PAGES: dict[str, tuple[str, str]] = {
         "Policy",
         "Capability and role policy surface. Placeholder until a later phase.",
     ),
-    "/insights": (
-        "Insights",
-        "Aggregate operational insights and trends. Placeholder until a later "
-        "phase.",
-    ),
 }