Fix all static analysis errors (golangci-lint)

- Fix errcheck: handle all error return values in production code
- Fix errcheck: handle all error return values in test files
- Fix staticcheck: replace deprecated WithCredentialsFile with WithAuthCredentialsFile
- Remove unused code: authHeaders, planToEatPlannerItem, planToEatResponse
- Use defer func() { _ = x.Close() }() pattern for ignored close errors

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

1 files changed, 2 insertions, 2 deletions

diff --git a/internal/handlers/response.go b/internal/handlers/response.go
index 3976f02..9a7ab45 100644
--- a/internal/handlers/response.go
+++ b/internal/handlers/response.go
@@ -10,7 +10,7 @@ import (
 // JSONResponse writes data as JSON with appropriate headers
 func JSONResponse(w http.ResponseWriter, data interface{}) {
 	w.Header().Set("Content-Type", "application/json")
-	json.NewEncoder(w).Encode(data)
+	_ = json.NewEncoder(w).Encode(data)
 }
 
 // JSONError writes an error response as JSON
@@ -32,5 +32,5 @@ func HTMLResponse(w http.ResponseWriter, tmpl *template.Template, name string, d
 // HTMLString writes an HTML string directly
 func HTMLString(w http.ResponseWriter, html string) {
 	w.Header().Set("Content-Type", "text/html")
-	w.Write([]byte(html))
+	_, _ = w.Write([]byte(html))
 }