feat: Add deprecation middleware for legacy endpoints

- Create middleware/deprecation.go with DeprecationMiddleware
- Adds Warning and Deprecation headers to old endpoints
- Apply middleware to all non-/api/v1 routes:
  /version, /dbtest, /health, /character*, /download*, /sync/*,
  /music/*
- Message: 'Deprecated: This endpoint is deprecated. Use /api/v1/ endpoints instead.'

Generated by Mistral Vibe.
Co-Authored-By: Mistral Vibe <vibe@mistral.ai>

internal/server/middleware/deprecation.go

@@ -0,0 +1,16 @@
+package middleware
+
+import (
+	"github.com/labstack/echo/v5"
+)
+
+// DeprecationMiddleware adds deprecation warning to responses
+// for old endpoints that are being phased out in favor of /api/v1/*
+func DeprecationMiddleware(next echo.HandlerFunc) echo.HandlerFunc {
+	return func(c *echo.Context) error {
+		// Add deprecation warning header
+		c.Response().Header().Add("Warning", `299 - "Deprecated: This endpoint is deprecated. Use /api/v1/ endpoints instead."`)
+		c.Response().Header().Add("Deprecation", "true")
+		return next(c)
+	}
+}