feat: add customer messaging interface in shop

- Add shop messages API endpoints (/api/v1/shop/messages)
- Add shop messages page routes (/shop/account/messages)
- Add messages.html template for customer account
- Add Messages card to account dashboard with unread badge
- Customers can view/reply to vendor_customer conversations

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

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

app/routes/shop_pages.py

@@ -581,6 +581,65 @@ async def shop_settings_page(
     )
 
 
+@router.get("/account/messages", response_class=HTMLResponse, include_in_schema=False)
+async def shop_messages_page(
+    request: Request,
+    current_customer: Customer = Depends(get_current_customer_from_cookie_or_header),
+    db: Session = Depends(get_db),
+):
+    """
+    Render customer messages page.
+    View and reply to conversations with the vendor.
+    Requires customer authentication.
+    """
+    logger.debug(
+        "[SHOP_HANDLER] shop_messages_page REACHED",
+        extra={
+            "path": request.url.path,
+            "vendor": getattr(request.state, "vendor", "NOT SET"),
+            "context": getattr(request.state, "context_type", "NOT SET"),
+        },
+    )
+
+    return templates.TemplateResponse(
+        "shop/account/messages.html", get_shop_context(request, db=db, user=current_customer)
+    )
+
+
+@router.get(
+    "/account/messages/{conversation_id}",
+    response_class=HTMLResponse,
+    include_in_schema=False,
+)
+async def shop_message_detail_page(
+    request: Request,
+    conversation_id: int = Path(..., description="Conversation ID"),
+    current_customer: Customer = Depends(get_current_customer_from_cookie_or_header),
+    db: Session = Depends(get_db),
+):
+    """
+    Render message conversation detail page.
+    Shows the full conversation thread.
+    Requires customer authentication.
+    """
+    logger.debug(
+        "[SHOP_HANDLER] shop_message_detail_page REACHED",
+        extra={
+            "path": request.url.path,
+            "conversation_id": conversation_id,
+            "vendor": getattr(request.state, "vendor", "NOT SET"),
+            "context": getattr(request.state, "context_type", "NOT SET"),
+        },
+    )
+
+    return templates.TemplateResponse(
+        "shop/account/messages.html",
+        get_shop_context(
+            request, db=db, user=current_customer, conversation_id=conversation_id
+        ),
+    )
+
+
 # ============================================================================
 # DYNAMIC CONTENT PAGES (CMS)
 # ============================================================================