fix: security hardening and HTTP status code corrections

- importSTRMHandler: add URL scheme validation (blocks file://, ftp://, etc.)
- backup Delete/Restore: harden path traversal check (block backslash, require .db extension)
- HTTP 201 for create endpoints: register, subscription, download client, notify channel, library, STRM import
- Error handling: return 500 for service/infra errors in download client and notify channel handlers

Co-Authored-By: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>

internal/handler/auth.go

@@ -65,7 +65,7 @@ func registerHandler(svc *service.Container) gin.HandlerFunc {
 			c.JSON(http.StatusBadRequest, gin.H{"error": err.Error()})
 			return
 		}
-		c.JSON(http.StatusOK, gin.H{
+		c.JSON(http.StatusCreated, gin.H{
 			"user":   u,
 			"tokens": tokens,
 		})