Standardize UI error logging: consolidate repeated 'failed to load' blocks

[cawalch]

Problem

12+ instances of "failed to load ... for UI" logging are scattered across 9 handler files with inconsistent APIs:

Inconsistent logging calls

File	Line	API used
ui_sources.go	155	h.logger().ErrorContext(ctx, ...)
ui_sites.go	137	h.logger().Error(...) (no ctx)
ui_iocs.go	76	h.logger().ErrorContext(ctx, ...)
ui_jobs.go	1253	h.logger().ErrorContext(ctx, ...)
ui_secrets.go	33	h.logger().Error(...) (no ctx)
ui_alerts.go	129, 158, 219	h.logger().ErrorContext(ctx, ...)
ui_dashboard.go	264	h.logger().Error(...) (no ctx)
ui_alert_sinks.go	63, 97	h.logger().Error(...) / h.logger().Warn(...)
ui_allowlist.go	23	h.logger().Error(...) (no ctx)

Some use ErrorContext(ctx, ...) with structured context, others fall back to Error(...) without the request context — making traceability inconsistent.

Impact

• Telemetry gap: Without ctx, logs lack request tracing metadata (request ID, correlation ID).
• Inconsistent UX: Some errors render via RenderError, others use raw http.Error(w, ...).
• HTMX awareness missing: Error responses don't consistently account for HTMX partial vs full-page requests.

Proposed fix

Expand the existing error_renderer.go / RenderError to cover list handler errors:

// LogAndRenderListError logs a structured error and renders an appropriate response.
func LogAndRenderListError(
  ctx context.Context, logger *slog.Logger, w http.ResponseWriter, r *http.Request,
  err error, msg string, basePath string, pageMeta PageMeta,
) {
  logger.ErrorContext(ctx, msg, "error", err)
  RenderError(ctx, w, r, err, basePath, pageMeta)
}

Replace all 12+ instances with a single call. Standardize on ErrorContext everywhere.

Files to change

• services/merrymaker-go/internal/http/error_renderer.go — expand helpers
• All 9 handler files listed above

Acceptance criteria

• All error logging uses ErrorContext(ctx, ...) consistently
• Single helper in error_renderer.go handles log + render for list errors
• No behavior change to end users' error experience
• Lint + tests pass

[cawalch]

Opened PR #192 to standardize the remaining list-load error logging paths through a shared helper. Verified with: cd services/merrymaker-go && go test ./...