feat: add admin messaging interface

- Add admin messages API endpoints (/api/v1/admin/messages)
- Add admin messages page route (/admin/messages)
- Add messages.html template with split-panel conversation view
- Add messages.js Alpine component for messaging UI
- Add Messages link to admin sidebar under Platform Administration
- Add message icon with unread badge to admin header
- Update init-alpine.js with headerMessages component

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/routes/admin_pages.py

@@ -476,6 +476,79 @@ async def admin_customers_page(
     )
 
 
+# ============================================================================
+# NOTIFICATIONS ROUTES
+# ============================================================================
+
+
+@router.get("/notifications", response_class=HTMLResponse, include_in_schema=False)
+async def admin_notifications_page(
+    request: Request,
+    current_user: User = Depends(get_current_admin_from_cookie_or_header),
+    db: Session = Depends(get_db),
+):
+    """
+    Render notifications management page.
+    Shows all admin notifications and platform alerts.
+    """
+    return templates.TemplateResponse(
+        "admin/notifications.html",
+        {
+            "request": request,
+            "user": current_user,
+        },
+    )
+
+
+# ============================================================================
+# MESSAGING ROUTES
+# ============================================================================
+
+
+@router.get("/messages", response_class=HTMLResponse, include_in_schema=False)
+async def admin_messages_page(
+    request: Request,
+    current_user: User = Depends(get_current_admin_from_cookie_or_header),
+    db: Session = Depends(get_db),
+):
+    """
+    Render messaging page.
+    Shows all conversations (admin_vendor and admin_customer channels).
+    """
+    return templates.TemplateResponse(
+        "admin/messages.html",
+        {
+            "request": request,
+            "user": current_user,
+        },
+    )
+
+
+@router.get(
+    "/messages/{conversation_id}",
+    response_class=HTMLResponse,
+    include_in_schema=False,
+)
+async def admin_conversation_detail_page(
+    request: Request,
+    conversation_id: int = Path(..., description="Conversation ID"),
+    current_user: User = Depends(get_current_admin_from_cookie_or_header),
+    db: Session = Depends(get_db),
+):
+    """
+    Render conversation detail page.
+    Shows the full conversation thread with messages.
+    """
+    return templates.TemplateResponse(
+        "admin/messages.html",
+        {
+            "request": request,
+            "user": current_user,
+            "conversation_id": conversation_id,
+        },
+    )
+
+
 # ============================================================================
 # INVENTORY MANAGEMENT ROUTES
 # ============================================================================